III. ENVIRONMENTAL BASELINE

The environmental baseline for this assessment includes effects of past and ongoing human and natural factors leading to the current status of the species or its habitat and ecosystem (FWS 1994). Additional baseline information on hydrology and species abundance and distribution is provided in Sections II and IV, respectively.

A. Historic and Present Biological Communities on the Lower Colorado River

1. Introduction

Prior to development, the Colorado River flowed unimpeded some 1,700 miles with a vertical elevation drop of more than 14,000 feet from its beginnings in the southern Rocky Mountains and eastern Great Basin to its terminus at the Gulf of California (Ohmart et al. 1988). The lower portion of the river from the Grand Canyon downstream was typically low gradient and flowed through a rather broad alluvial valley with relatively few confined reaches. At its mouth was an alluvial delta containing vast marshes, riparian forests and backwaters. Such habitats were present along the entire reach of the lower river. At its mouth was an alluvial delta containing vast marshes, riparian forests and backwaters. Such habitats were present along the entire reach of the lower river. The riparian belt extended away from the river for up to several miles where the water table was relatively shallow.

Historically, the seasonal hydrograph and tremendous sediment loads associated with the lower Colorado River were dominating factors driving the physical and biological attributes of the ecosystem. Recorded flows at Yuma ranged from 18 cfs to 250,000 cfs with sediment loads averaging more than 10⁸ metric tons per year (USGS 1973).

Seasonal flooding resulted in the creation of several distinct communities of plants and animals. High water occurred around June with low flows occurring during the winter months. Riparian communities were in a constant state of succession as the river, on a seasonal basis, was constantly depositing new sediment, shifting its channel, and creating and destroying habitat. Floodplain communities developed in areas that were seasonally, or only intermittently, inundated. Marsh communities developed in areas prone to extended periods of inundation, and the aquatic community evolved consisting of a main channel with separate or connected oxbows and backwaters.

The overall ecosystem of the lower Colorado River today is quite different from that which existed prior to modern day use and development. Table 4 summarizes the chronology of the lower Colorado River development which has, in part, resulted in the current ecosystem.

2. Riparian Communities

a. Historic

Although the historic riparian communities along the lower Colorado River were dynamic, human-induced change since the beginning of the century has resulted in an ecosystem having significantly different physical and biological characteristics. Such changes have taken place as a result of the introduction of exotic plants (such as saltcedar), the construction of dams, river channel modification, the clearing of native vegetation for agriculture and fuel, fires, increasing soil salinity, the cessation of seasonal flooding, and lowered water tables. Figure 24 illustrates an example of the change in vegetation communities from 1879 to 1977.

The hydrology of the river created a series of terraces and bottoms along its route. Grinnell (1914) identified seven river associated communities. Five of these were specifically flood-plain in nature including: 1) Cottonwood-Willow association; 2) Arrowweed association; 3) Quail-bush association; 4) Mesquite association; and 5) Saltbush association. Two other communities, the River and Tule association, are also discussed (Ohmart et al. 1988). Figure 25 illustrates typical historic floodplain terraces and associated vegetation communities occurring along the lower Colorado River. Figure 26 illustrates a reconstruction of historic native plant community placement and principal species composition from original surveyor notes and plats along the lower Colorado River in 1879 [ The General Land Office, now known as the Bureau of Land Management, initiated the original township surveys or cadastral mapping along the river in 1855. Not all the land was surveyed during the same period of time. Figure No.26 shows a reconstruction of the general vegetative types below Blythe, California in 1879 derived by interpreting floral descriptions contained in original field notebooks and then transferring these to the original field plats (Ohmart et. al., 1977 in Importance. Preservation and Management of Riparian Habitat: A Symposium, USDA Forest Service, General Technical Report RM-43)] .

The first terrace of the river was dominated by cottonwood (Populus fremontii) and Goodding willow (Salix gooddingii). Associated under-story shrubs were dominantly seepwillow (Baccharis sp.) (Ohmart et al. 1988); stands of cane (Phragmites sp.) were also common. Occasionally screwbean mesquite, although rare, was interspersed among willows in areas subject to infrequent flooding. Vegetation types found here were adapted to a frequently flooded environment, with their life-cycle often dependent upon flooding for successful regeneration. They tended to be short-lived and fast growing.

Seasonal floods resulted in the deposition of new sediment beds which served as nursery areas for new stands of cottonwoods and willows. Deposition of sediment was facilitated by plant growth which acted to slow water velocities across the floodplain. On other areas of the terrace, the floodplain was just as rapidly eroding away.

With respect to wildlife, the cottonwood-willow association was probably the most important biotically. Grinnell (1914) noted 66 species of birds and 12 species of mammals, most of which were transient and migrant species. Grinnell noted that almost all of the birds listed for the willow association were either insectivorous or raptorial. Granivorous (grain eating) species were notably absent. The greater part of the passerine birds were transients or winter visitants. Only 3 were observed as permanent residents. The deer mouse (Peromyscus maniculatus) was the only observed rodent of wide and plentiful occurrence.

Three other rodents occurred locally, notably the cotton rat (Sigmodon sp). Otherwise, the only other mammals noted by Grinnell in the cottonwood-willow association were far-ranging predators. The paucity of terrestrial mammals in this association was probably due to the repressive effect of the annual overflow.

Arrowweed communities were found in a band along the outer margin of the cottonwood-willow communities where inundation was not as extreme. It typically produced an almost monotypic stand dense enough to preclude growth of other vegetation. Grinnell collected 14 bird and 6 mammal species here. The only bird that achieved noticeable abundance was the song sparrow (Melospiza melodia).

Honey mesquite (Prosopsis glandulosa) formed almost monotypic stands, with some associated undergrowth. The roots of honey mesquite are capable of reaching water tables up to 50 feet deep. Stands formed along the second terrace (see Figure 25) in areas that escaped flooding for a number of years, because the plant is intolerant of inundation. The second terrace typically occurred at an elevation of 4 to 18 feet above the river channel (Minckley and Brown 1982). This area was typically considered the outer edge of the riparian zone. More than 30 species of birds and 9 species of mammals were observed using this vegetative association, with mesquite providing both food and shelter (Grinnell 1914). Mesquite produced beans, which provided an important source of food for many species of wildlife, and served as a host for mistletoe which attracted insects and produced an abundant crop of berries. Several of the winter and resident birds of this association depended almost wholly on these berries (Grinnell 1914). Grinnell (1914) also observed 4 species of birds which bred in this association: Albert’s Towhee, Crissal Thrasher, Lucy’s Warbler, and Phainopepla. One mammal, the woodrat (Neotoma albigula), was commonly observed by Grinnell in the mesquite association.

The final riparian vegetative association was the saltbush or quail-bush association. It also formed almost monotypic stands or was associated in clumps with mesquite. Monotypic stands, when formed, tend to be clumped. As a whole, this area of the riparian zone was rather sparsely vegetated.

Creosote bush appeared quite often within this zone, although not achieving dominance until further away from the floodplain. Grinnell (1914) observed 13 species of birds and 3 species of mammals in this association. Although important for its food values, as seed production in this habitat type was abundant, additional values existed as escape and breeding cover for both birds and mammals. Quail and cottontail rabbits were often observed taking refuge in the tangled mass as did bush-inhabiting sparrows (Grinnell 1914).

b. Present

The system currently used to classify vegetation along the lower Colorado River is based on plant community and structural type (Anderson and Ohmart 1984). Six structural types have been described (I to VI) and refer to the proportion of foliage present in each of three vertical layers. For example, a plant community with structural type VI has most of its foliage in the lowermost layer, less foliage in the mid-height layer, and little or no foliage in the upper canopy. A structural type I community has well-developed foliage in all three layers, with the upper canopy dominating. Figure 27 illustrates the relationship between the six structural types and the foliage density at various heights. Community and structural types correlate with wildlife habitat quality, especially for birds; generally type VI provides the poorest habitat and type I the best.

Reclamation has mapped the distribution and acreage of the different riparian plant communities along the lower Colorado River since 1976 (Anderson and Ohmart, 1976; Anderson and Ohmart, 1984, Younker and Anderson, 1986). Updated maps, compiled from 1994 aerial photography, will be finalized in 1996. It must be stressed, however, that although the 1994 aerial photography covered the entire river from Davis Dam to the United States-Mexico border, the entire width of the floodplain was not flown in all places so that coverage is only approximately 80 percent of the previous efforts (John Carlson and David Salas, USBR, pers. comm.). Direct comparisons between previous acreages and 1994 acreages may not be applicable, especially for community and structural types prevalent at the extreme outer portions of the floodplain.

As of 1986, the lower Colorado River floodplain supported 107,749 acres of riparian, marsh, and desert vegetation between the United States-Mexico border and Davis Dam. This includes 45,037 acres of saltcedar; 5,754 acres of cottonwood-willow [ Criteria used in classifying this community type (cottonwood-willow) included the presence of ( Salix gooddingii ) and ( Populus fremontii ) (the latter in extremely low densities) and where such species constitute at least 10 percent of total trees.] ; 1,683 acres of honey mesquite; 15,492 acres of screwbean mesquite; 7,880 acres of saltcedar and honey mesquite association; 8,930 acres of arrowweed; quail-bush and inkweed; 12,549 acres of marsh vegetation; and 426 acres of creosote scrub (Younker and Andersen 1986).

The most abundant community/structural types observed in 1986 (Younker and Andersen 1986) were, by far, saltcedar type IV (22,381 acres) and saltcedar type V (17,560 acres). Honey mesquite type IV consisted of 8,889 acres, saltcedar-screwbean mesquite type IV consisted of 7,825 acres, arrowweed type VI consisted of 7,478 acres, and quail-bush type VI consisted of 1,231 acres. A complete description of the 1986 community and structural type acreages found along the lower river (per River Division) is shown in Table 5.

Preliminary data from the 1994 flight shows a change in the acreage and structure of certain riparian plant communities (John Carlson and David Salas, USBR, pers. comm.). There is a net loss of approximately 2,300 acres of cottonwood-willow along the lower Colorado River below Davis Dam. This loss is almost entirely from the IV-VI structure classes which regenerated during the high flows of the early 1980s. Some of these young stands have survived and grown into structure type III stands while others were out competed by saltcedar. Contrastly, cottonwood-willow types I-III have increased by over 1,300 acres below Davis Dam, with an additional 1,100 acres now present in the Lake Mead delta near Pierce Ferry, Arizona (Figure 28). This represents an over four fold increase in cottonwood-willow types I-III below the Grand Canyon. A similar trend may be observed in saltcedar structure types I-III. Riparian plant communities comprised of mesquite and/or saltcedar are difficult to compare between 1986 and 1994 because they are found throughout the floodplain and are especially prevalent at the outer extremes which the 1994 mapping effort did not adequately cover. A description of the 1994 data is shown in Table 6.

Since Grinnell’s 1910 survey of the lower Colorado River, numerous additional surveys and investigations concerning the biotic attributes of the lower river system have been conducted. Probably one of the most recent and comprehensive terrestrial descriptions can be found in the Reclamation-funded Wildlife Use and Densities Report of Birds and Mammals in the Lower Colorado River Valley (Anderson and Ohmart 1977). This report describes the average densities and diversities of birds and mammals as associated with the 26 vegetative community and structural types mentioned above. The information given in this report was obtained from data collected over a 4-year period, and involved continuous year-round surveys in each of the habitat types from Davis Dam to the Mexican border, near Yuma, Arizona. Over 250 species of birds and approximately 15 species of mammals were observed during this survey. Generally, the survey showed the highest bird and mammal densities and diversities in cottonwood-willow with mesquite, mesquite-saltcedar (mix) and saltcedar communities, respectively lower. Thus, the diverse structural types I and II had the greatest species richness while the least diverse structure types V and VI had the lowest richness.

3. Marsh

a. Historic

With the exception of the lower Colorado River delta area, historic evidence suggests that backwater marshes that lasted several years seldom were very large along the lower Colorado River. Freeland (1973) stated that before completion of Parker and Imperial Dams, marshes along the river below Davis Dam were 1,000 acres or less in area. Grinnell (1914), as quoted in Ohmart et al. (1975), stated:

"The river’s habit of overflow would be expected to result in rather extensive tracts of palustrine flora. As a matter of fact, however, marshes were few and of small size. This was probably due to the rapid rate of evaporation of overflow water so that favoring conditions did not last long, and also to the rapid silting-in of such water basins as ox-bow cutoffs. As a result there were either almost lifeless alkali depressions, or lagoons practically identical in biotic features with the main river. But in a few places there were well-defined palustrine tracts kept wet throughout the year, chiefly by seepage. These were always located back from the river near the outer edges of the broader valleys, where they were least affected during flood time. They were marked by growths of tules, sedge, and saltgrass, sometimes the latter alone, and were usually surrounded by arrowweed or willow associations."

Grinnell (1914) conducted a 3-month survey along the lower Colorado River. While the survey period was brief, and certainly couldn’t include seasonal use of the river by the fauna, it is one of the few scientific biological surveys available to draw from for pre-development conditions of the river. Even by the time Grinnell conducted his survey, modification of the lower Colorado River had started. Diversions were in place north of Blythe, California, and at Laguna Dam, near Yuma, Arizona.

Observations by Grinnell in marsh communities (tule association) included 9 species of birds and 4 species of mammals. He noted that water birds, with the exception of herons, egrets, and bitterns, did not remain along the lower Colorado River to breed, and that the only mammal of abundance was the western harvest mouse. Other, less abundant, mammals included the hispid cotton rat, muskrat, and raccoons.

With regard to avian fauna, observations by Grinnell include only stragglers of white pelicans and the brown pelican was not seen at all. Cormorants were not seen until the party was in the vicinity of Laguna Dam, and they were seen only in small numbers. The blue heron was abundant along the whole course of the river from Needles to Yuma. The green heron was seen 5 miles above Laguna and was common from that point to Pilot Knob. The common egret was seen in only one place; the recently silted-in area above Laguna Dam. Fifty black-crowned night herons were seen in a pond below Ehrenberg; the species was also common farther down the river. Killdeer were scarce and the spotted sandpiper was occasionally sighted. The belted kingfisher occurred only as a migrant.

b. Present

Present-day marshes along the lower Colorado River are of two kinds. The first kind includes backwater marshes, which are defined as marsh areas adjacent to the river and which are either directly connected to the river or are connected by seepage. The second kind, which is more extensive, includes those marshes formed by impoundments such as the marshes in Mittry Lake, Imperial Reservoir, Lake Havasu, Topock Marsh, and other similar impounded areas.

The construction of river control features, such as training structures, along the lower ColoradoRiver has resulted in the formation of more permanent and expansive backwater marshes. There are over 400 backwater marshes along the lower Colorado River today from Davis Dam to Laguna Dam. Some of these marshes were created and are maintained specifically for mitigation for channel improvement projects. Reclamation actively pursues maintenance and restoration of backwater marshes not tied to mitigation on a cost shared basis. These backwater marsh habitats are subject to successional factors as were the historic marshes along the river. Under normal operating conditions, this succession is greatly slowed because current river conditions and operating criteria result in less scouring and associated sediment movement. Bankline stabilization has reduced erosion and associated sediment accrual to the river. When exceptional conditions are encountered, such as the high flow releases which occurred in 1983-1985, channel scouring occurs with associated sediment deposition in those backwater areas. These exceptional conditions would be expected to promote accelerated succession to upland conditions which are dominated by saltcedar (Tamarix sp.).

The majority of the banklines of the flowing river have been stabilized. This does not allow for natural marsh formation resulting from the river channel moving laterally, which would occur during high flows. Additionally, current river operating criteria reduce the opportunity for high flows (floods) which would also reduce natural marsh formation during those type of flows. A portion of the backwater marshes, which exist along the river today, are isolated from the main river channel, reducing the opportunity for flushing flows through them. However, it was observed during the high flows experienced on the river during 1983 through 1985, the isolated backwater marshes did not fill in with deposited sediment. Impacts which occurred to those isolated backwater marshes were a result of the main river channel scouring and the resulting drop in water table. In any case the marsh communities formed, as a result of the impoundments and training structures, are much greater in extent and permanence than those which occurred historically. As stated above, some of these marshes are specifically maintained for fish and wildlife purposes.

In 1986, the lower Colorado River floodplain supported over 12,000 acres of marsh associated habitat (Table 5). Younker and Anderson (1986) classified the marsh communities into six different types based primarily on the percentage of cattail, bulrush, common cane and open water (Table 7). Of the total 12,000+ acres of marsh habitat found, nearly 50 percent (5,657 acres) was classified as type 1 which met the criteria of being nearly 100 percent cattail/bulrush with small amounts of common cane and open water. For descriptions concerning the remaining amounts and type of marsh habitat observed by Younker and Andersen refer to Tables 5 and 7.

Vegetation mapping being completed in 1996 shows the lower Colorado River floodplain supporting a little over 11,000 acres of marsh habitat. Of this amount, 4,216 acres were classified as type 1, down about 25 percent from 1986. This was to be expected, as the high flows from 1983 through 1985 had created additional marsh area. Upon the return to normal flows, these areas reverted back to terrestrial.

In addition to 1986 type maps, Reclamation funded a 1986 report describing the development of a fish and wildlife classification system for backwaters found along the lower Colorado River from Davis Dam to Laguna Dam (Holden et al. 1986). The 2.5 year study effort resulted in over 400 backwater areas being identified and classified. The backwaters were characterized by State, distance from the SIB, river division, how formed (natural or man-made), quality of associated riparian vegetation, how accessible, size, how connected to the river, shape, permanence and actual acreage of open water.

After classifying the backwaters, seasonal field studies were then undertaken to sample fish and wildlife distribution, abundance, and preferences. Eighteen individual backwaters were sampled. These efforts included sampling water quality, zooplankton, benthic macro invertebrates, and fish in nine fishery study backwaters. Wildlife studies on the 18 backwaters also included morning bird censuses, night spotlighting, small mammal trapping, and aerial waterfowl surveys. Over 100 avian species, 25 mammal species and 15 fish species were observed, quantified, and associated with classified backwaters.

4. Aquatic

a. Historic

Historically, the lower Colorado River represented a unique aquatic habitat, ranging from a swift-flowing, turbid river during the annual runoff period (May-July) with flows exceeding 100,000 cfs to a gentle meandering river during late fall and winter periods with flows of 5,000 cfs or less (Grinnell 1914; Carothers and Minckley 1981). Remarkably high sediment loads accompanied floods and seasonal runoff from the Rocky Mountains. Sediment loads averaged more than 200 million metric tons per year during 1925 through 1935 leaving the Grand Canyon, but only 140 million metric tons made it as far as Yuma, Arizona. In all but those places where the river breached hard-rock barriers, the bottom continuously shifted as bedload was transported (Minckley 1979). Where the stream occupied broad alluvial valleys, sediment was deposited and wide, shallow, braided channels developed. As meanders matured, they were cut off to form oxbow lakes and backwaters. Extensive, although transitory, marshes were formed, only to be obliterated by vegetative succession, or more rapidly destroyed by currents and transported sediments during floods (Minckley 1979). Some of the larger historic backwaters and/or oxbows were persistent enough to be given names, these included Beaver Lake, Lake Su-ta-nah, Duck Lake, Spears Lake, Powell Slough (now part of Topock Marsh), and Lake Tapio. All were located between present day Bullhead City and Topock (Ohmart et al. 1975).

Along the lower river, changes in stage or water elevation were of two types. The first was the annual rise and fall due to flooding from snowmelt runoff. Because the river carried so much sediment, a film of silt clearly marked the annual high water level. Grinnell (1914) remarked on how tremendously high the silt line was up on the canyon walls in the more restricted reaches of the river. During floods, the wide valley sections, such as those near Blythe, California, and again at Cibola, Arizona, were flooded as the river overflowed its banks. Because of the great sediment loads, depressions were often filled in so that when the water receded, it left a relatively flat plain.

The second type of change to water surface elevation was at best unpredictable and was related to both the constant shifting of channels, as the river deposited sediments, and local storms. Grinnell (1914) states:

"The time of lowest water is in midwinter, that of highest flood in June...while throughout the year fluctuations of less extent are liable to occur at any time."

As an indication of just how rapid these flow changes were, Grinnell (1914) made the following statements in regards to trying to trap beavers:

"Further troubles in our efforts to trap beavers resulted from continual rising or falling of the river and from the heavy deposit of silt....At the last trapping place, the lowering of the water repeatedly exposed both the traps and stakes...."

Another indication of rapid water level changes is gleaned from Grinnell’s (1914) observations on great blue herons during his 1910 surveys of the lower Colorado River:

"Along the whole course of the river...blue herons were almost continually in sight. Their chief foraging grounds were the mud bars traversed by shallow diversions of the river. The habit of the river of having frequent periods of falling water, even when, as in the spring, the aggregate tendency is to rise, results in the stranding of many fishes in the shallow overflows as the water seeps away or evaporates. This frequently recurring supply of fish appears to be the chief source of food of all the species of herons occurring in the region."

Water temperatures fluctuated seasonally with the water being warmer in spring and summer and cooler during fall and winter. Physico-chemical regimes varied with the flow regimes; i.e., spring run off, summer flooding, or conversely, summer drought. Temperatures most likely resembled those today in reaches far from mainstream reservoirs. Nighttime water temperatures decline rapidly as a result of evaporation into dry air and respond quickly to intense insolation at sunrise. Daytime water temperatures were often 86°F in low-elevation, desert streams but approached104°F when insolation was accompanied by high relative humidities (Deacon and Minckley 1974). Chemical conditions, prior to closure of dams, must have been almost as highly variable as discharge. De-oxygenation of deeper parts of many backwater habitats occurs today and must have prevailed locally in the past. Total dissolved solids were high, especially during drought, and Sykes (1937) recorded a fish kill in the lower Colorado River as a result of an influx of "alkali" waters from the Gila River in the late 1800s.

Productivity in the main river channel was low, with the food-web being primarily detrital based (Minckley 1979). High turbidity limited the development of phytoplankton, and the shifting silt substrate inhibited development of benthic algal, macrophytes and invertebrate communities. Riverine organisms subsisted primarily on detrital drift from upstream or the surrounding riparian communities. Autotrophic production was limited to river margins or in backwaters, sloughs, and oxbows lateral to the main channel where sediments dropped out, allowing light to penetrate the water column. Grinnell (1914) credited main channel turbidity and sediment load for the lack of aquatic weeds along the lower river, and therefore the sparsity of waterfowl. He inferred the high importance of backwater sloughs to herons and fish was related to the lack of turbidity there:

"For reasons already explained there is relatively little cryptogamic aquatic flora in the Colorado River. There is therefore little or no food-supply from this source to attract plant-eating ducks. This category of water-birds was, in fact, very sparsely represented. On the other hand, herons were notably plentiful because of the supply of catfish and carp made abundant at intervals by the drying-up of overflow ponds. While fishes were not abundant in the main stream, they were plentiful in backwater sloughs, where, the water was more nearly clear because the sediment had a chance to settle out." (Grinnell 1914)

Litter fall from the surrounding riparian zone was another important source of nutrients driving production in backwater areas (Minckley 1979). Fish often moved in and out of backwaters, where the sunlight and nutrients allowed for the development of invertebrate and plankton communities (Seethaler 1978, Vanicek 1967). These habitats served as nursery areas for many fish species.

The historic fish community of the lower Colorado River was de-depauperate (in a species richness sense), probably representing a combination of effects from the Pleistocene ice-age, tremendous environmental fluctuations, and complete isolation from other drainages (Miller 1959). The historic fauna had one of the highest levels of endemism of any river basin in North America (Miller 1959). There were 10 native species of fish in the lower river.

Three species were marine in origin (Minckley 1979). They include the spotted sleeper (Eleoteris picta) (only 1 specimen has ever been catalogued), the Pacific tenpounder (machete) (Elops affinis) (typically estuarine but does ascend into the lower reaches of many rivers), and the striped mullet (Mugil cephalus) (another estuarine species often found in the lower river, at times, in quite large numbers). None of these species’ ranges extended much past the location of the current Imperial Dam (Minckley 1979).

Desert pupfish (Cyprinodon macularius) were found in the lower reaches of the Colorado and Gila Rivers into the early 1900s. These fish inhabited backwaters and springs lateral to the river margins (Minckley 1979). No known collections of this species were made in the Colorado River upstream of Yuma (Dill 1944).

Six other species historically occurred in the river: bonytail (Gila elegans), roundtail chub (Gila robusta), Colorado squawfish (Ptychocheilus lucius), razorback sucker (Xyrauchen texanus), flannelmouth sucker (Catostomus latipinnis), and woundfin (Plagopterus argentissimus) (Minckley 1979).

The roundtail chub probably occupied the lower river but was only collected sporadically. There were historical records from around Yuma, Arizona, but the fish were not believed to have been abundant (Minckley 1973). Roundtail chub typically inhabit smaller streams, such as the Salt, Verde and Virgin Rivers, and Moyle (1976) suggested that the fish captured around Yuma were rare stragglers from upstream tributaries. (Two roundtail chub captured above Imperial Dam in 1973 were described by Minckley [1979] as being heavily blotched in coloration typical of the populations of the Bill Williams River, an upstream tributary.)

Presence of the woundfin in the lower Colorado River may similarly reflect downstream displacement due to floods. Records indicate specimens of woundfin were collected near Yuma around the turn of the century, but no fish collections from the mainstem Colorado River have reported this species since. Today the species is limited in distribution to the Virgin River of Utah, Arizona, and Nevada (FWS 1995).

At one time flannelmouth suckers occurred in the entire lower river, but were very limited in numbers. Minckley (1973) suggests their distribution probably paralleled that of the razorback sucker and Colorado squawfish. The native population was most likely extirpated; however, a population of 600 was transplanted from the Paria River, a tributary to the Colorado River above Lake Mead, to the mainstem Colorado River below Lake Mohave by AGFD in 1976. That population has survived and exists today.

The remaining three fishes, Colorado squawfish, bonytail, and razorback sucker, made up the majority of the historical fish assemblage along the lower Colorado River. (The lower Gila River had a similar makeup of fishes.) Literature reviews suggest Colorado squawfish and razorback suckers were, at times, incredibly abundant, and bonytail were widespread and encountered frequently (Minckley 1979). However, probably owing to the dynamic nature of the river, wide fluctuations in reproductive success probably occurred. All three of these species are long lived and produce high numbers of young. These life-history factors allow for stability in the populations or at least persistence of adults when drought or other conditions do not allow for successful production and recruitment over a period of years.

Habitats used by these three fishes were variable. Body forms, such as dorsal keels on razorback suckers and narrow caudal peduncle on bonytail, were indicative of swift water habitats, but food availability was low in the main channel. Energy needs were high in swift water areas, especially during warm periods. Intuitively then, slack water areas such as oxbows, isolated channels, backwaters and flooded lowlands were important feeding areas for razorback sucker, bonytail,and Colorado squawfish. These habitats also provided refuges for protection, feeding, and growth of young fishes (Minckley 1979). During flooding, adults of all three species could avoid harsh current conditions by moving into these areas.

The migration habits of these fishes during predevelopment times are not known for certain, but can be hypothesized for at least Colorado squawfish and razorback sucker using data from recent studies (Tyus 1985; Bestgen 1990). (Data for bonytail are lacking.) Colorado squawfish in the Green River of Utah and Colorado have extensive spawning migrations (Tyus and McAda 1984) and must have had similar spring migrations for spawning in the lower Colorado River basin. Minckley (1979) proposed that there were fall migrations of Colorado squawfish to the lower river to feed on annual runs of striped mullets. This conclusion was based on the large numbers of Colorado squawfish reported in the lower river and the otherwise depauperate food supply. (Intuitively, a large number of predatory fishes could not be sustained unless adequate food resources were available.)

Razorback sucker are suspected of moving great distances to spawn, but field data are not definitive (Minckley et al. 1991). Data suggest that the fish are randomly dispersed prior to the spawning and move to spawning grounds but, at times, move also between spawning grounds (Minckley et al. 1991). Recent sonic tracking of razorback sucker in Lake Mohave show this same pattern. Adult fish were randomly distributed around the lake prior to spawning season and then migrated to spawning areas during January through March. Some of the tagged fish even visited two or more spawning aggregations during the same season, moving back and forth across the lake (G. Mueller and P. Marsh pers. comm.).

b. Development Along the Lower Colorado River

The present aquatic ecosystem of the lower Colorado River is tremendously different than that just described. These changes began in the late 1800s. The human populations of the Colorado River Basin States grew rapidly during the mid-to-late 1800s as people immigrated from the eastern United States and from other countries. Unfortunately for the native fishes of the American southwest, they did not come alone. The Colorado River basin, with its endemic fish community isolated for thousands of years, was invaded and swamped with new species in a very short period of time. Just as unfortunate for the native fishes, the growing human population set out to tame and harness the Colorado River, building flood control dams, storage reservoirs, and agricultural diversions. These two concurrent actions, introduction of non-native fishes and dam building, are described chronologically below.

The first fish "invader" was the common carp, Cyprinus carpio. Originally an Asiatic species, the carp reached California in August 1872, when a Mr. J.A. Poppe brought fish from Holstein, Germany, to Sonoma Valley. He raised them in ponds and sold the offspring in the western states, Hawaii, and Central America (Calhoun 1966). The species was stocked into Utah waters in 1881 (Sigler and Miller 1963), into Nevada waters in 1881 (Allan and Roden 1978), and into Arizona waters sometime prior to 1885 (Minckley 1973). The species was reported from the Colorado River basin before 1900 (Gilbert and Scofield 1898 in Minckley 1973). When and how the carp gained access to the Colorado River is not specifically known, but it most likely occurred in the 1880s. This was the period when the U.S. Fish Commission was championing carp as a table food. H.G. Parker, first Fish Commissioner for Nevada, stated in his 1881 biennial report, "One of my great aims has been to stock our waters with the best species of carp,..." (LaRivers 1962). Unfortunately, he attained his goal.

Channel catfish (Ictalurus punctatus) is the next documented exotic fish introduction for the lower Colorado River. Unlike the carp, this species is native to North America, commonly occurring in the Mississippi River drainage. The species was introduced into California in 1874 (Calhoun 1966) and into Utah in 1888 (Sigler and Miller 1963). The species was stocked into the lower Colorado River by the Arizona Fish Commission 1892 when 722 adult and yearling fish were released (Worth 1895 in LaRivers 1962).

The first large scale water diversion project for agricultural purposes on the lower Colorado River was the construction of the Alamo Canal. The Imperial Valley area around the Salton Sink was noted as a fertile valley with great agricultural potential back in the 1850s, but it needed water. A canal project was conceived then, but it didn’t begin until 1895. The canal alignment was along the United States-Mexico border, and when constructed, much of the canal actually was in Mexico. The canal was completed in 1901, delivering water to Imperial Valley via diversion at Rockwood Gate. By 1904 more than 12,000 people had moved to the area and bought land for agricultural purposes through government auctions. However, the sediment load of the Colorado River was greatly underestimated, and by the end of 1904 the Alamo Canal was blocked with sediment and the Imperial Valley was again without water. To remedy this, a temporary diversion of the Colorado River was constructed at the United States-Mexico border. During a local flood which came down the Gila River in October 1905 this diversion failed and the entire flow of the Colorado River rushed into the Salton Sink. The break in the dike was repaired by the Southern Pacific Railroad Company in February 1907 (USBR 1981). Flowing of the entire Colorado River discharge into the Salton Sink for 16 months filled the Salton basin to an elevation of 195 feet below sea level. The Salton Sink which had been intermittently dry during most of the 19th century but had contained flood waters in 1828, 1840, 1852, 1859, 1862, 1867, and 1891 had not reached such a level for at least the last 300 years (Littlefield 1966). By 1925 the Salton Sea had dropped over 50 feet, to about -250 ft. msl.

The desert pupfish (Cyprinodon macularius) was most likely the only native fish in the Salton Sink prior to formation of the Salton Sea. (San Felipe Creek and Salt Creek, which were perennial, are suspected to have been occupied habitat.) Freshwater and marine fishes from the Colorado River entered the sink with the flood waters. A CFG information bulletin on the Salton Sea published in 1978 reported that the razorback sucker, bonytail chub, carp, and catfish were initial inhabitants, along with the striped mullet and the Pacific tenpounder (Hulquist et al. 1978). No mention was made of the Colorado squawfish, but it was most likely also present.

Being in a closed basin and located in a severe desert habitat help explain the rapid increase in salinity which occurred between 1907 and 1920. Initial solution of minerals from the floor of the Salton Sink, plus dissolved minerals in the agricultural runoff and drainage and the continuous evaporation of the surface water, combined to cause the salinity of the Salton Sea to increase to that of ocean water (35 parts per thousand) by 1920. Most of the fresh water fishes died off, leaving only the desert pupfish and striped mullet as abundant fishes. From 1929 to 1953 California introduced numerous sport fishes and other aquatic organisms into the Sea, including striped bass, silver salmon, halibut, anchovy and bonefish. All of these introductions failed except for the orange mouth corvina, sargo, and bairdiella, three marine species still present today. With salinity continuing to rise in the Salton Sea (now over 43 parts per thousand) it is speculated that sometime in the future the desert pupfish will again be the only fish inhabiting the Salton Sea.

Laguna Dam, located thirteen miles north of Yuma, Arizona, and roughly 8 miles upstream from the mouth of the Gila River, was the first structure to block the entire river channel on the lower Colorado River. Authorized in 1904 to provide diversion for the Fort Yuma Indian Reservation and the communities of Yuma, Arizona, and Bard, California, Laguna Dam was completed in 1909. The hydraulic height of the dam was only 10 feet (the difference in water surface elevation on upstream and downstream sides of dam). Dill (1944) described Laguna Dam as, "...passable to fish because of its low height and some breaks in it." Grinnell (1914) surveyed the flora and fauna around Laguna Dam in 1910, one year after it was completed, and made the following observations regarding Colorado squawfish:

"A huge minnow (Ptychocheilus lucius), called locally "Colorado salmon," ...was plentiful immediately below Laguna dam, where many were being taken by the Indians living near there."

His remarks infer that Laguna Dam may have blocked passage of Colorado squawfish or at least provided a concentration point for their capture by local Indians.

Mosquitofish (Gambusia affinis) are native to the central United States and were widely introduced during the 1920s and 1930s for mosquito control. This species was introduced into California in 1922 and reported from the Salton Sea area between 1927 and 1929. The fish was widely spread along the lower river when Dill conducted his survey in 1942 (Dill 1944).

Largemouth bass (Micropterus salmoides) was the next exotic species to enter the Colorado River system. The species was stocked into California in 1874 (Calhoun 1966), into Utah in 1890 (Sigler and Miller 1963), and into Nevada waters by 1900 (LaRivers 1962), but it is unclear just when it was introduced into the lower Colorado River. Dill (1944) mentioned the uncertainty of its origin, but inferred it predated Hoover Dam in the following statements:

"The species has been planted several times in waters of the Colorado, and the existing stock undoubtedly has a multiple origin. Although present for many years, according to "old-timers," it did not become plentiful until the water cleared."

The "water cleared" with the closing of Hoover, Parker, and Imperial Dams in the late 1930s.

The Boulder Canyon Project Act of 1928 authorized two actions that forever altered the lower Colorado River. The first was the construction of Hoover Dam which occurred from 1931 to 1935. This was the first high dam on the river.

The most obvious change brought to the lower Colorado River by Hoover Dam was the trapping of the sediment by Lake Mead. Estimated to be as much as 200,000,000 metric tons annually, the sediment load of the river quickly dropped behind the massive structure. Lake Mead was expected to trap 137,000 acre-feet of sediment annually. As the reservoir filled, it was predominantly clear water, as most of the sediment dropped out in the lower reaches of Grand Canyon and in the rapidly forming delta at the head of the lake.

FWS (then the Bureau of Sport Fisheries) took advantage of these conditions by stocking game fishes including largemouth bass, bluegill sunfish (Lepomis macrochirus), green sunfish (Lepomis cyanellus), and black crappie (Pomoxis nigromaculatus) into Lake Mead (Allan and Roden 1978). Lake Mead quickly gained national recognition as a great sport fishery when a 13 lb. 14 oz. largemouth bass won first place in the 1939 Field and Stream nationwide fishing contest (Wallis 1951).

At Hoover Dam the discharge was clear and cool. The river, freed from its sediment load because of the upstream reservoir, attacked the stream bed, removing sand and other fine sediments. This allowed for the introduction of another new sportfish, the rainbow trout (Oncorhynchus mykiss) FWS began stocking rainbow trout in 1935. Jonez and Sumner (1954) described the changing aquatic habitat and developing trout fishery in the Hoover Dam tailrace as follows:

"Rainbow trout first were introduced below Hoover Dam in 1935. By 1937, the swift current below Hoover Dam had scoured the sand away from the gravel and rubble, leaving the water crystal-clear for a distance of about four miles below the dam. The first trout were being caught by 1940. By 1941, about 18 more miles of gravel and rubble had been scoured clean of sand. By 1947, the clear water extended about 42 miles below Hoover Dam....Between 1935 and 1951, a total of 3,714,054 rainbow trout was planted in the area which now is Lake Mohave."

The second action authorized by the Boulder Canyon Project Act of 1928 was the construction of the All-American Canal System which included the All-American Canal, Coachella Canal, and Imperial Dam and Desilting Works. Canal construction began in 1934 and was completed in 1940. Imperial Dam construction began in 1936 and was completed in 1938. Coachella Canal construction began in 1938 but was interrupted by World War II and was not completed until 1954.

Imperial Dam spanned the river just 5 miles upstream of Laguna Dam and formed the head works for two canals. On the California side, desilting works were constructed for the intakes of the All-American Canal, and the Arizona side contained the headgates and desilting works for the Gila Gravity Main Canal. These two canal systems have a design diversion capacity of over 17,000 cfs.

Unlike Hoover Dam and Lake Mead, Imperial Dam did not form a large, deep impoundment nor did it discharge clear water. However, it did cause considerable backing up of the water, and the formation of wide shallow lakes lateral to the main channel such as Ferguson Lake on the California side and Martinez Lake on the Arizona side. In these areas the water was clearer and quieter and good sport fisheries did develop.

In 1934, the same year construction began on Imperial Dam, work began on Parker Dam which formed Lake Havasu. Similar to Hoover Dam, Parker Dam blocked sediment flow, released clear water, blocked upstream and downstream migration of native fishes, became populated with numerous introduced fishes (both game and nongame) and generally continued the alteration of the historical aquatic ecosystem of the lower Colorado River. One feature associated with Parker Dam that was not heretofore seen along the lower Colorado River was the pumping plant constructed for MWD for its Colorado River Aqueduct. Accounts described in Dill (1944) suggested that the pumping plants removed large quantities of fish (albeit exotic) from Lake Havasu.

Headgate Rock Diversion Dam was completed in 1941 about 14 miles downstream of Parker Dam by the U.S. Indian Service to provide irrigation water to the CRIT Reservation near Parker, Arizona. This "run-of-the-river" structure has no real storage capacity.

Construction of Davis Dam in Pyramid Canyon some 67 miles downstream of Hoover Dam was begun in 1946 and completed in 1953. This formed Lake Mohave, a reregulation reservoir designed to meet the requirements of the Mexico Treaty of 1944.

Sometime between 1948 and 1953, red shiners (Cyprinella lutrensis) gained access to the lower Colorado River, probably as a baitfish release. It was being reared at that time as a bait fish at a private fish farm near the Colorado River in Ehrenberg, Arizona (McCall, 1980). NDOW and AGFD jointly stocked this species into Lake Mohave in 1955 (Allan and Roden 1978).

Threadfin shad (Dorsoma petenense) was introduced into Lake Mead in 1953 (Allan and Roden 1978), and into Lake Havasu in 1954 (Calhoun 1966) and quickly spread throughout the lower river system. Calhoun (1966) describes just how quickly this species took hold in the lower Colorado River basin:

"Only two plantings, totaling 1,020 fish, were made in Lake Havasu. These threadfin and their off-spring populated the entire Colorado River from Davis Dam southward to the Mexican border, the Salton Sea, and related irrigation canals within 18 months."

As the threadfin shad became abundant, state game and fish agencies decided to make use of the new forage base by stocking another predatory fish, the striped bass (Morone saxatilis). Between 1959 and 1964, CFG made 19 separate stockings of this species between Davis and Imperial Dams, totaling over 100,000 fish. Most of these fish came from the Tracy Fish Screen near Stockton, California, at the intake to the Central Valley Project canal (Guisti and Milliron 1987). The species was stocked into Lake Mead in 1969 (Allan and Roden 1978).

The next nonnative fish introduction into the lower Colorado River was that of the African mouth brooder, the blue tilapia, Tilapia aureau. (A number of species of the genus Tilapia have been introduced and are not easily separated in the field. This group of fishes is herein referred to as "tilapia.") These fish were thought to feed on aquatic plants and were introduced for weed control in the irrigation systems. AGFD raised tilapia at its Bubbling Ponds facility near the Page Springs Hatchery and stocked these fish throughout the State between 1961 and 1980 (Grabowski et al. 1984). A breeding population of Tilapia mossambica was found in a smallpond near the Salton Sea in 1964 (St. Amant 1966 in Grabowski et al. 1984). CFG stocked Zilli’s tilapia or redbelly tilapia into irrigation canals around Blythe, California, during the 1970s (Grabowski et al. 1984). Tilapia are common in the lower reaches of the river. No confirmed collections have occurred upstream of Parker Dam, although the fish is abundant in Alamo Reservoir on the Bill Williams River, a tributary to Lake Havasu.

Flathead catfish (Pylodictis olivaris) was first reported in Arizona from the Gila River basin in the 1950s (Minckley 1973). It was stocked into the lower Colorado River by AGFD in 1962 (McGinnis 1984). The species had spread upstream to Parker, Arizona by 1976 (Minckley 1979), and it was observed in Lake Havasu in 1984 (USBR file data).

The CAP began construction of intake facilities on the southeast end of Lake Havasu in 1973. The Havasu Pumping Plant lifts water over 800 feet to the start of a 335-mile long aqueduct. In full operation the Havasu Pumping Plant has a capacity of 3,000 cfs. The CAP will deliver an average of 1.5 maf of water each year to cities, Indian communities, industries and farmers.

c. Effects of Development and Present Day Aquatic Baseline

Today, the lower Colorado River downstream of Grand Canyon is a tremendously diverse aquatic ecosystem with over 240,000 surface-acres of open water (Table 8). There are over 27 fish species occupying habitats ranging from deep, clear reservoirs to turbid, flowing river, to warm shallow marshes. While the system on an overall basis is diverse, meaning one reach of river does not look like the next, individual reaches do not change much from season to season. The annual changes in the system are missing. Historically the river environment could be described in one word, extreme! The river annually went from hot to cold, and from raging flood to gentle tranquility. Today, reservoirs are clear and deep all year long. For example, over two-thirds the volume of Lake Mead remains at 55 degrees 12 months of the year, resulting in a constant, cool discharge at Hoover Dam. Even in the lower reaches of the Colorado River between Blythe, California, and Yuma, Arizona, where the river is turbid and shifting sand beds still occupy the river bottom, annual fluctuations in discharge and sediment load are almost immeasurable when put on a scale with the historical ranges of these parameters. Even the daily water level changes, which occur below almost every dam, are constant and rhythmic. Unlike conditions described by Grinnell (1914), whereby rapid changes in water levels trapped fish in shallow pools and side channels (to the benefit of herons), stranding of fishes under the current operational release patterns are extremely rare and virtually non-existent.

Table 8. Surface acreage of water along the lower Colorado River from Pierce Ferry to the U.S./Mexico International Boundary by river maintenance division (Water Classification).

DIVISION	FLOWING RIVER	RESERVOIR	BACKWATER	TOTAL
Lakes Mead & Mohave	0	191,500	20	191,520
Mohave	3,554	0	3,767	7,321
Topock Gorge	1,183	0	239	1,422
Havasu	515	20,510	740	21,765
Parker	3,748	0	1,364	5,112
Palo Verde	2,350	0	160	2,510
Cibola	1,971	0	505	2,476
Imperial	3,154	560	2,608	6,322
Laguna	436	25	585	1,046
Yuma	1,782	0	82	1,864
Limitrophe	0	0	146	146
TOTALS	18,693	212,595	10,216	241,504

The native fishes were adapted to the system of extremes. They spawned early, before the peak runoff, and their developing young moved into off-channel areas along with the rising flood waters to feed and grow. Migrations up or downstream were possible due to their body forms, and their long life allowed them to persist when reproductive failure occurred for successive years due to drought or other calamities. While top carnivores where included in the community, species such as the razorback sucker hid during the day and grew quickly to sizes less vulnerable to predation. The introduced fishes such as carp and catfish quickly invaded the off-channel habitats as witnessed by Grinnell (1914) who found them abundant in backwaters along with bonytail and razorback sucker. As discussed by Dill (1944), the physical extremes of the river system prior to dam construction must have been equally hard on native and nonnative fishes alike, and although these exotic fishes were present, their numbers were not great.

Dill (1944) reported that the populations of native fishes had declined prior to 1930. He proposed that native fishes were at a low point in their respective populations just prior to the period of dam building and that nonnative fish populations rapidly expanded with the taming of the river and prevented the rebuilding of native stocks. In his own words:

"...it seems probable that the native fish populations have undergone alternate periods of rise and fall. But each period of destruction was followed by a period during which the population could rehabilitate itself.... Because of the unfavorable water conditions around the early thirties it seems possible that the population of native fishes sank to one of its low points and that the coincidental advent of clear water following Boulder Dam brought about a heavy production of bass and other alien fishes which preyed upon the already reduced natives."

Dill (1944) argued that the native fishes had a high biotic potential which had allowed them to bounce back from previous catastrophes and had it not been for the presence of exotic fishes, they would have done so.

Minckley (1979) similarly argues that dam construction alone was not sufficient to destroy the native fish communities of the lower Colorado River:

"Destruction of the native fauna of the lower Colorado River has been attributed to physical modifications of the environment, such as channelization and construction of dams.... Considering the great age of the Colorado River, and correspondingly great ages of at least some of the genera of fishes inhabiting it..., sufficient time has been available for them to have experience far more change than has recently been effected by man.

"Excluding special cases..., almost all declines in native fish populations are directly attributable to predation by small adults or juveniles of introduced kinds upon early life- history stages of indigenous forms. Shoreline and backwater habitats once exclusively available to non-piscivorous juveniles of suckers and minnows now are inhabited by mosquitofish and young centrachids, and cropping by those animals destroys the native fauna."

Clearly, destruction of the native fauna was not a one-time event. It took some time, and in the case of razorback sucker and possibly bonytail, it is still going on today. In Lakes Mead, Mohave, and Havasu native fish expanded their populations along with the expanding aquatic habitat as the water bodies filled. Jonez and Sumner (1954) described the spawning of both bonytail and razorback sucker in Lake Mohave and of razorback sucker in Lake Mead (detailed later in this volume in accounts of each species). LaRivers (1962) details spawning of razorback sucker in Lake Havasu in 1950.

One of the few observations made of large numbers of juvenile razorback sucker this century was made in Lake Mohave in 1950, and it serves here to demonstrate how these fish populated new reservoirs during initial filling. In describing the habitat used by razorback sucker, Sigler and Miller (1963) state the following:

"This large sucker is an inhabitant of large rivers and has adjusted well to the impoundments of the lower Colorado River Basin.... The young occur in shallows at the river or reservoir margins where individuals approximately an inch long travel in schools numbering thousands. Over 6,000 specimens were taken in two hauls of a minnow seine at the margin of the Colorado River in Nevada on June 15, 1950. Here the temperature was 71-76 degrees F, whereas the adjacent river was only 58 degrees."

Davis Dam closed and began storage in January 1950. According to statements by Minckley et al. (1991), the above cited capture of juvenile razorback sucker occurred at Cottonwood Landing, which is approximately 21 miles upstream of Davis Dam. The quoted information suggests that the reservoir had backed up to that point, because the differences stated in water temperature between the riverine and ponded areas is similar to what is found today at the inflow of the Colorado River to the lake some 20 more miles upstream.

It seems apparent that as the new water bodies filled, native and nonnative fish were initially successful in recruiting young into adulthood. As time went on, the nonnative populations were able to prey on the eggs and young of native fishes and recruitment into adulthood all but ceased for the native fishes. Adults continued to survive until they succumbed to natural causes, which in the case of razorback sucker took upwards of 50 years.

Further data supporting the hypothesis that the native fishes were initially successful in recruitment were presented by McCarthy and Minckley (1987). They analyzed otoliths of seventy Lake Mohave adult razorback suckers killed between 1981 and 1983. Roughly 88 percent hatched prior to or coincident with construction and filling of Lake Mohave (1942-1954).

Ongoing work in the upper Colorado River basin, regarding the role of flooded bottom lands in the ecology of razorback suckers, provides just as striking information on how quickly the nonnative fishes can overshadow such recruitment. In attempts to increase natural recruitment of native fishes, FWS personnel flooded a bottom land parcel with water from the Green River, near Vernal, Utah, during the spring of 1995. At the end of the summer, they drained the wetland and found 28 young razorback suckers. These were the first young razorback suckers of this size observed in that age group since 1965. Unfortunately, they only represented a very small portion of the fish in the wetland. Of the 11 tons of fish measured, 95 percent were non-natives. Carp dominated the catch by weight, and fathead minnows (Pimephales promelas) were numerically the most abundant fish species (FWS 1995).

In the lower Colorado River of today, physical and chemical conditions do not favor the nonnative fishes over the native fishes, except for possibly lack of turbidity. Adequate water quality exists in the form of water volume, water temperature, dissolved oxygen, pH, specific conductance, hardness, etc. for reproduction, nursery, rearing/growth, and resting for native and nonnative fishes. Spawning habitat in the form of clean hard substrates are excessively abundant in both lentic and lotic reaches (relative to pre-Hoover Dam period). Primary production is adequate to sustain tons of fish production. Chlorophyl levels range from 1.0 to 5.0 mg/l (Paulson and Baker 1984), which is remarkably normal for fresh waters in the temperate zone world wide (Taylor et al. 1980). Zooplankton levels in mainstem reservoirs are on the order of 10 to 50 individual organisms per liter, a level typically found in temperate lakes across North America. Benthic invertebrates in riverine reaches are probably one or two orders of magnitude greater than that which occurred in the main channel Colorado River prior to Hoover Dam. Macrophytes abound in many reaches of the lower river, adding to the already high autotrophic production. So why do the native fish not survive?

The main problem is the sheer number of new species, all with reproductive potentials as great or greater than the native fishes. Taking the three most common native fish, (historically) razorback sucker has roughly 100,000 eggs per female, Colorado squawfish produce about 100,000 eggs per female, and bonytail produce roughly 50,000 eggs per female (Hammond pers. comm.). One of each species would yield 250,000 eggs per spawning season. Female carp average 500,000 eggs (Carlander 1969), striped bass in the lower Colorado River have over 500,000 eggs (Edwards 1974), one channel catfish produces 10,000 eggs (Carlander 1969), largemouth bass average 40,000 (Carlander 1977), one bluegill sunfish yield 25,000 eggs (Carlander 1977), one green sunfish produces 25,000 eggs (Carlander 1977), black crappie average 50,000 eggs (Carlander 1977), and even one four inch threadfin shad yields 10,000 eggs per year (Carlander 1969). One of each would total over one million for one year. Multiply these numbers by the factor of differential survival (e.g. catfish and sunfish guard their young in nests while the three native fish are broadcast spawners) and the picture becomes clearer. The nonnative fish quickly out produce the native fish. And while not all of these immature fish survive, the greatest number of each species present are the young fish (young of year and yearlings) which are the primary predators on young native fishes.

In Lake Mohave, Jonez and Sumner (1954) observed razorback sucker and bonytail (separate observations) spawning in large groups and the adults did not protect their eggs and larvae. In each observation, carp were observed feeding on the eggs, and young bass and/or sunfish were observed with the larvae.

Juvenile native fishes also succumb to predation. Marsh and Brooks (1989) report on the stocking of juvenile razorback suckers into the Gila River in Arizona between 1984 and 1986. They released 35,475 fish in three separate stockings. They concluded that channel catfish and flathead catfish within the first 40 kilometers of river downstream from the release sites were able to remove the entire population of planted fish.

One possible explanation for this high incidence of catfish predation was provided by the NFWG on Lake Mohave. Its work showed the juvenile razorback sucker to be nocturnal in habit, seeking protective cover during daylight hours. These observations suggest that juvenile suckers attempted to hide in the same cavities occupied by catfish, inadvertently seeking out the predator (USBR file data).

In summary, the aquatic ecosystem that exists in the lower Colorado River today, and forms the aquatic baseline for this BA, is highly modified and is physically, chemically, and biologically different than that which existed historically. Native fishes are mostly extirpated or endangered of becoming so. Physical modifications by dam construction and reservoir formation have homogenized the river system, effectively removing the "extremes" to which only the native fishes were adapted. Without such extremes the native fishes have no advantage over nonnative fishes and both groups are able to express their reproductive potential as regards to release of gametes. Differential mortality on native fishes due to predation on early life stages by nonnative fishes sufficiently suppresses the recruitment of native fish to the adult life stage and in a matter of only a few generations, extirpation is achieved. The primary limiting factor for recruitment of native fishes in the lower Colorado River basin today is nonnative fish predation on young life stages. This has been conclusively proven by the myriad of studies and experiments in which native fishes have been successfully reared in habitats from which nonnative fishes have been removed and excluded.

Recognizing this fact, a number of current conservation and recovery actions are being taken in the lower Colorado River basin by Reclamation and other agencies to raise native fish in protected, predator-free environments until they are big enough to avoid most predators occurring in the lower Colorado River. (These programs are described elsewhere in this document.) Similarly, fishery biologists in the upper Colorado River basin now recognize the problems caused by the invasion of nonnative fishes made possible because of dams and diversions and other developments along the Green and Colorado Rivers and their tributaries and are developing strategic plans to control nonnative fishes. Recent actions in the upper basin also include offsite rearing of native fishes and stocking of juveniles back into the river system.

B. Previous and Ongoing Section 7 Consultations

1. Colorado River Mainstem

Since 1973, Reclamation has informally and formally consulted under Section 7 of the ESA for various projects that potentially may have had direct or indirect effects on threatened and endangered species and critical habitat along the lower Colorado River (Table 9). Although the projects have varied substantially, as have the impacts, FWS has concluded that the projects would not jeopardize the continued existence of any species or its critical habitat. In some consultations, incidental take was provided by reasonable and prudent measures (RPMs). These consultations are considered as part of the environmental baseline in this document.

2. Baseline Projects

In addition to Reclamation activities that were evaluated for direct or indirect effects on the mainstream of the Colorado River, section 7 consultation and NEPA compliance have been completed or is in the process of being completed for authorized projects that provide facilities for the States to divert and distribute State waters confirmed by previously discussed court decrees. The CAP and Robert B. Griffith Water Project (southern Nevada) are summarized below as part of the environmental baseline.

a. Central Arizona Project Havasu Diversion

The CAP was constructed to provide a long-term, non-groundwater, water source for municipal, industrial, and non-Indian and Indian agricultural users in Arizona. The CAP was authorized for construction under the Colorado River Basin Project Act, Public Law 90-537 (82 Stat. 885), approved September 30, 1968. An approximately 330-mile long series of open canals, inverted siphons, pumping plants and tunnels convey water diverted from Lake Havasu on the Colorado River east through Phoenix and then south to the southern boundary of the San Xavier Indian Reservation southwest of Tucson. Under normally expected water supply conditions, project diversions from the Colorado River are expected to be about 1.5 maf per year of Arizona’s basic annual entitlement of 2.8 maf.

Reclamation has consulted formally and informally on over 50 CAP-associated projects. In April of 1994, after 3 years of intensive formal consultation with Reclamation, FWS issued a final BO on the Transportation and Delivery of Central Arizona Water to the Gila River Basin (Hassayampa, Aqua Fria, Salt, Verde, San Pedro, middle and upper Gila Rivers, and associated tributaries) in Arizona and New Mexico. The opinion found that deliveries of CAP water would jeopardize the continued existence of the spikedace (Meda fulgida), loach minnow (Tiaroga cobitis), Gila topminnow (Poeciliopsis occidentalis), and razorback sucker and would adversely modify the critical habitat of the spikedace, loach minnow, and razorback sucker. Reclamation is now in the process of implementing the reasonable and prudent alternatives (RPAs) presented in the opinion. Reclamation’s Phoenix Area Office is also preparing a biological assessment on the delivery of water into the Santa Cruz River Basin.

The Havasu Intake and Pumping Plant is located at the lower end of Lake Havasu downstream of the Bill Williams River Delta and within the Havasu National Wildlife Refuge.

The Havasu Pumping plant has the capacity to lift 2.2 maf per year of Colorado River water 800 vertical feet to the Hayden-Rhodes Aqueduct. Each of the six pump units has a capacity of 500 cfs. Trash racks with openings 6 x 16 inches cover the pump intakes, and predicted water velocity in front of the trash racks is 1.1 feet per second.

Reclamation’s Havasu Intake EIS (January 1973) addressed native, rare, and endangered species, concluding that "...very few fish in comparison to the overall fish population on Lake Havasu will move through the intake channel and be adversely affected by pumping operations. These fish would be types oriented to open water movement and feeding, such as threadfin shad and striped bass." The EIS stated that there would be a monitoring program to assess losses of fish and other aquatic biota in Havasu and "...data obtained in this initial phase and subsequent phases will be evaluated to determine whether protective measures are required." The emphasis at that time was clearly on sport fishes. The Fish and Wildlife Coordination Act (FWCA) Report from FWS, dated June 30, 1976, also recommended studies to determine the extent of any fishery losses. At the time of the EIS and the FWCA report neither the bonytail nor the razorback sucker were on the endangered species list.

In 1989, FWS, AGFD, and Reclamation submitted their report on the Lake Havasu Fishery Study. Sampling was conducted on either side of a half-mile long dike that forms an enbayment leading to a cement-lined channel and the pumping plant. Seasonal sampling was conducted from the spring of 1984 to December 1985. No razorback suckers were found during this study. However, adult razorback suckers were observed in the CAP canal in 1986 (Mueller 1989). The potential effects of entrainment and diversion on this species is discussed in Section IV of this document.

b. Southern Nevada Water System (Robert B. Griffith Water Project)

An environmental assessment was prepared in 1992 to obtain a contract for the uncontracted remainder of Nevada’s 300,000 acre-feet per year consumptive use apportionment. Section 7 compliance was concluded through informal consultation. By memorandum dated February 21, 1992, the FWS concurred with Reclamation’s determination that the proposed action was not likely to adversely affect the threatened desert tortoise.

Improvements to the Southern Nevada Water System (SNWS) were identified in the 1994 Final Environmental Assessment of the Colorado River Commission's Proposed SNWS Facilities Improvement Project. The improvements are associated with existing facilities. As part of the environmental compliance, Reclamation entered into formal section 7 consultation with FWS on August 31, 1994, for the Mojave desert tortoise, a federally listed threatened species. On December 6, 1994, FWS rendered its BO that the SNWS Improvement Project is not likely to jeopardize the continued existence of the threatened Mojave population of the desert tortoise and no proposed critical habitat will be destroyed or adversely modified. Incidental take was issued with RPMs to minimize take.

A draft EIS for the proposed Southern Nevada Water Authority Treatment and Transmission Facility (SNWA-TTF) was provided for public review and comment in November 1995. A final EIS is expected by December 1996. Reclamation initiated formal consultation on the desert tortoise on August 15, 1995, and received a draft BO on December 18, 1995. Because of a number of project refinements, Reclamation requested a number of extensions to incorporate these changes into the final BO. The additional information and comments were provided to the FWS on June 26, 1996, and a final BO is expected by September 1996. The draft BO found that the proposed project is not likely to jeopardize the continued existence of the threatened Mojave population of the desert tortoise and no critical habitat will be destroyed or adversely modified. Incidental take was proposed with RPMs to minimize take.

3. Salton Sea and Endangered Desert Pupfish

During the public review of the draft BA, concerns were raised regarding the status of the endangered desert pupfish in the Salton Sea area. A summary of past ESA consultations is provided below.

Following listing of the desert pupfish as an endangered species in 1986, a BO was issued by FWS (June 18) on the effects of agricultural drain maintenance on this species. The opinion found that both agricultural drain maintenance activities by IID and CVWD and the introduction of sterile grass carp would not jeopardize the continued existence of desert pupfish. The opinion allowed for unlimited incidental take of the species during drain maintenance.

When the desert pupfish was listed as an endangered species (March 31, 1986), critical habitat was designated for the species along San Felipe Creek/San Sebastian Marsh, an intermittent stream and marsh complex on the west side of the Salton Sea. Reclamation purchased all of the private land holdings within the critical habitat area for $300,000 and turned this land over to CFG under a quitclaim deed in 1990.

In June 1992, a second BO was issued regarding drain maintenance and its affect on desert pupfish. The consultation involved the Salton Sea National Wildlife Refuge drains maintained by IID. The opinion again found that the drain maintenance would not jeopardize the desert pupfish; however only a limited incidental take was allowed due to recent observations of increased pupfish populations in the drains. This opinion also covered effects on Yuma clapper rails and California brown pelicans. Similar to the desert pupfish, FWS was of the opinion that drain maintenance would not jeopardize the continued existence of either species.

C. Non-Federal (Contemporaneous and Cumulative) Actions

The environmental baseline also includes State, local, and other human activities that are contemporaneous with the consultation in process, while cumulative actions involve future State or private activities, not involving Federal activities, that are reasonably certain to occur in the action area. The various categories of these non-Federal activities are summarized in Table 10, while the diversion and use of State waters by principal entitlement holders for 1993 are summarized in Table 11. A detailed accounting of lower Colorado River water diversions, returns, and consumptive use is provided in the "Calendar Year 1995 Compilation of Records in Accordance with Article V of the Decree of the Supreme Court of the United States in Arizona v. California Dated March 9, 1964" (Appendix I). It is anticipated that these contemporaneous non-Federal actions will continue in the future, and the potential effects of such actions are referenced for each ESA-protected species in Section IV. Additionally, these cumulative actions will be addressed in the MSCP process.

Many non-Federal activities (Tables 10 and 11), dealing with the direct use of mainstem water and resulting from the diversion of water from the mainstem, have affected or may affect the natural resources of the lower Colorado River and its extended environs. These can be classified as impacts occurring 1) on the mainstem river or its reservoirs, 2) on the river’s floodplain, or 3) away from the river and its floodplain primarily due to the long-distance conveyance and use of Colorado River water.

In response to comments on the draft BA, Pilot Knob and Siphon Drop Powerplants are a special diversion feature of the All-American Canal that involves non-Federal power production and delivery of a portion of Mexico’s Treaty water. The diversion essentially routes the water around the upper portion of the Yuma Division resulting in reduced flows in the river channel. The amount of water diverted depends on available canal capacity in the All-American Canal and the amount of Mexico’s water order at any given time. The river in the Yuma Division is reduced in flow from Laguna Dam to the California wasteway (the outfall for water through the Siphon Drop Powerplant). At that point the river starts gaining water from Siphon Drop Powerplant to the Pilot Knob wasteway. The river again gains in flow at the Pilot Knob wasteway and is then diverted for Mexico’s use at Morelos Dam. An expanded discussion of the operation of this diversion is found in Appendix D.

Table 10. List of non-Federal activities that affect or may affect the resources of the lower Colorado River and its extended environs.

Affecting the mainstem river and its reservoirs	• diversion of state entitlement waters • potential decrease in water quality by: - municipal effluent discharge - storm water runoff - agricultural drainage - recreational waste - other non-point discharges • trash accumulation • increased recreational use: - fishing - hunting - boating - swimming
Affecting the river’s adjacent floodplain	• agricultural development: - land conversion - pesticide applications - soil erosion/minimum tillage - cropping patterns that benefit certain species - land fallowing • municipal and industrial development: - land conversion - air pollution (dust, automotive and industrial emissions) - natural area management • trash accumulation: - solid waste disposal (landfills) • increased wildfire frequency - reduced native riparian habitat/saltcedar expansion •increased recreational use: - hunting - camping - hiking - off-road vehicles
Affecting areas away from the lower Colorado River and its floodplain	• agricultural development: - land conversion - pesticide applications - water pollution (of ground or surface waters) - soil erosion/minimum tillage - land fallowing - air pollution (dust and smoke from burning field residues) - cropping patterns benefitting some species - water conservation and reuse • municipal and industrial development: - land conversion - air pollution (automotive and industrial emissions) - water pollution (of ground or surface waters) - solid waste disposal (landfills) - water conservation and reuse • increased recreation: - resource impacts (off-road vehicles, trampling) - management plans - developed recreational sites

Table 11. Amounts and uses of water diverted by principal water entitlement users in 1993.

Diversion Project	Irrigation Supply (acre-feet of water)	Acres Irrigated	Municipal and Industrial Supply (acre-feet of water)	Population Served
Ak-Chin Indian Community AZ	72,239	14,655
Coachella Valley CA	304,174	58,579
Imperial Valley CA	2,677,597	461,642	45,410	99,610
Cibola Valley AZ	22,184	3,557	37	199
Lake Havasu AZ			12,666	32,144
MWD			1,207,329	15,000,000
Arizona, miscellaneous	10,798	1,952	34,384	56,140
Mohave Valley AZ	37,741	4,186	6,394	12,050
Nevada, miscellaneous			31,455	7,000
Central Arizona Project	384,425	143,641
Wellton-Mohawk (Gila Valley) AZ	291,817	56,814	876	1,605
Yuma Mesa (Gila Valley) AZ	316,743	33,360
Southern Nevada Water Project			295,120	854,565
Palo Verde Irrigation District CA	219,780 a	121,000a
Fort Mojave Indian Reservation CA, AZ, NV	129,767a	20,076a
Quechan Indian Reservation CA	51,616a	7,743a
Cocopah Indian Reservation AZ	9,707a	1,524a
Chemehuevi Indian Reservation CA	11,340a	1,900a
CRIT CA, AZ	717,148a	107,588a
TOTAL	5,257,076	1,038,217	1,633,671	16,063,313

^a[ Amount of entitlement; not necessarily amount actually diverted or irrigated.]