Platte River Ecosystem Resources and Management, with Emphasis on the Big Bend Reach in Nebraska
The Physical Setting
Geographic Location
The South Platte River originates as snowmelt in central Colorado at about 12,500 feet above sea level. From its source, the river flows southeastward, then north-northeastward, and, after crossing the Colorado-Nebraska border, flows almost due east to join the North Platte. The South Platte is about 450 miles long and drains about 24,300 square miles.
Also beginning as snowmelt, the North Platte River flows northward from north central Colorado into central Wyoming where it gradually curls to the southeast before joining the South Platte River in Nebraska. From its source at about 11,000 feet above sea level to its confluence with the South Platte, the North Platte River traverses about 665 miles and drains an area of 34,900 square miles.
The Platte River originates at the confluence of the North Platte and South Platte rivers near North Platte, Nebraska. From its source, the Platte River flows eastward along an S-shaped course and empties into the Missouri River near Omaha, Nebraska. Along this 310 mile route about 29,800 square miles are drained by the Platte River and its major tributaries (U.S. Fish and Wildlife Service (USFWS) 1981). The stretch of river above the Platte's confluence with the Loup River, between Grand island and Lexington, is commonly called the Big Bend Reach or central Platte.
Hydrology
Vast changes in the natural hydrograph are largely responsible for the changed nature of the Platte River basin ecosystem one sees today. Because long term hydrologic records prior to development are lacking, the history of irrigation has been developed using records of canal construction and other available information. Although water development has varied in time throughout the basin, the history of development consisted of four main stages: (1) construction of small, crude ditches to irrigate flood plains; (2) construction of larger canals to irrigate bench lands; (3) construction of reservoirs to store snowmelt runoff; and; (4) accelerated development of groundwater resources.
Eschner et al. (1983) described development of irrigation in the Platte River basin and the availability of flow records used to document hydrologic changes. They concluded that overappropriation of summer flows in the South Platte River basin occurred between 1880 and 1885, and varied in the North Platte River basin in Wyoming beginning in 1889 for the smaller tributaries and culminating in the main stem being overappropriated by 1901. Summer flows of the North Platte River in Nebraska were overappropriated between 1914 and 1917. They found few reservoir storage data for most of the reservoirs constructed in the basin prior to 1910, and concluded that the beginning of large-scale reservoir construction probably marked the date of streamflow overappropriation. The dates of overappropriation generally mark the end of stage two described above, and the unquantified effects of that water development are included in the flow recorded at the Overton gage. The flow record also includes the effects of approximately 0.7 million acre-feet and 2.0 million acre-feet of storage developed in the South Platte River and North Platte River basins, respectively. These water resource developments need to be kept in mind when assessing changes in flow records.
Pre-development spring runoff produced a hydrograph with a large seasonal variation, peak flows in May and June, and low base flow from August to March (Hydrology Work Group 1989). Before 1900, flows exceeded 20,000 cubic feet per second (cfs) almost every year (prior to 1909, flows in the North Platte alone averaged 19,400 cfs) (Currier et al. 1985). Paleo-floods in the central Platte River were probably in excess of 100,000 cfs. In 1965, the South Platte River flowed at 123,000 cfs at Balzac and at 37,600 cfs at Julesburg, Colorado, as a result of a storm that produced 466,000 cfs on Bijou Creek (Friedman et al. In Press). The historical record lists no accounts of overbank flooding. The earliest aerial photographs (1938), however, reveal that the surrounding wetlands were subject to overbank flows. The ability of the river channel to modify its cross section to increase conveyance at bankfull flows probably limited overbank flooding except at very high peak discharges.
Climatic events including droughts and wet periods also influence stream gage records in the Platte River basin. The source of supply for the Platte River basin is largely derived by snowfall accumulation and runoff from the Rocky Mountains in the headwaters of the North and South Platte Rivers. Based on tree ring studies and climatic, precipitation, and streamflow studies, Duever (1991) presented a pattern of "average" flows for at least a few decades prior to 1900, high flows until 1930, then very low flows during the 1930's, followed by a return to a more "average" range of flows sometime in the 1940's, which has continued at least into the early 1980's. These climatic variations are reflected in the stream flow gage at Overton in addition to the effects of water development. Therefore, any estimate of the "true" change in flow based on stream flow records is precisely that, an estimate.
Eschner (1981) reported no long-term change in climate during the past 80 years and speculated that climate in the eastern plains has been relatively constant for the last 200 years. Although a wet cycle was postulated for around the turn of the century for the Colorado River system (which probably also affected Platte River flows), the massive floods of the post-glacier era have not been repeated in recent history. Predictions of the 100-year flood return period have been roughly estimated by the U.S. Geological Survey at about 50,000 cfs (Lewis and Caughran 1958), but are probably conservative.
Some evidence suggests that the central Platte River in the Big Bend area occasionally experienced very low flow because of channel infiltration (Eschner et al. 1983). On the other hand, Miller (1978) stated that "indirect evidence such as construction of canals along the Platte to divert water during the summer months suggest that prior to irrigation the Platte River did not routinely go dry". Because of inconsistencies, records of stream flow measurements in the late 1800s and early 1900s created a confusing picture of pre-development low flows (Currier et al. 1985). As a result, the occurrence of historic no-flows in the Big Bend Reach prior to irrigation diversion in the mid-1800s cannot be described. No-flow days recorded in the post-1930s, however, may have been influenced by water diversion and storage upstream (Currier et al. 1985).
Zero historic flows were probably uncommon because of the extensive area of the Platte River drainage basin and the perennial flows of the numerous tributaries (Currier et al. 1985). Furthermore, ground water levels in the general area of the river respond very rapidly to changes in river stage (within 24 hours for distances up to 2,500 feet from the river), which ensures that some water was in the river at all times prior to 1860 (Hurr 1983). Limited gaging records from the pre-development era also suggest perennial flows. For instance, before major reservoir development (pre-1909), the mean annual base flow at Duncan was 970 cfs during August and September (Currier et al. 1985).
Almost all zero discharges reported by Bentall (1982) at Lexington, and many of those recorded at Columbus before 1909, were low flows (Currier et al. 1985). These low flows are the result of inaccurate measurements, probably a consequence of the river channel shifting away from the gage, or scouring or filling around the gage, or improper calibration of the gage for lower flow measurements (Currier et al. 1985). All eleven days reported by Bentall (1982) at Lexington as no-flow days in 1906, for example, actually had gage readings that had been recorded without a corresponding discharge.
Numerous historical references document the braided form of the Platte (Bradbury 1819, James 1923, Kelly 1851, Mattes 1969). In virtually every account the bed material is described as sand. James (1923) wrote "its bed is composed almost exclusively of sand, forming innumerable bars, which are continually changing their positions, and moving downward...the alluvial deposits of which the river bottoms are formed, consist of particles of mud and sand, more or less minute." This braided form was maintained by high peak discharges and a large sediment load. Pre-development sediment loads in the central Platte River can only be estimated. Historically, high peak flows on the North Platte (mean peak flow was 19,270 cfs at North Platte from 1895-1909) undoubtedly delivered substantial quantities of sediment which kept the river wide, shallow, and very active.
A thorough discussion of changes in channel geometry, river flow hydrology, and bed material size is found in Currier et al. (1985), O'Brien and Currier (1987) and Lyons and Randle (1988). The metamorphosis and current dynamics of the South Platte River are discussed by several authors (Knopf and Scott 1990, Friedman et al. In Press, Koch 1994, Nadler and Schumm 1981, Scott et al. In Press).
Based upon flow data from the Overton gauge, the mean annual flow in recent years has increased relative to the period before 1930, and is more variable (Figure 3). Mean annual flow prior to 1930 was in the range of 2.5-3.0 million acre-feet. Mean annual flow declined below 1.0 million acre-feet for the majority of 1930 through 1965, and subsequent to 1965 has varied between 0.7 and 2.8 million acre-feet.
Vegetation and Habitats
The Platte River valley is characterized by forest, shrub, and sandbar vegetation on the river floodplain; lowland prairie and cultivated fields on the river terraces; and upland prairies on the loess bluffs along the ancient river escarpment. The floodplain forest, shrub, and sandbar communities have developed on coarse-textured alluvial soils adjacent to the river channel. The forest communities have open canopies and are dominated by cottonwoods (scientific names of plants and animals are given in Appendices A-F) with an understory of red cedar and rough-leaf dogwood. Green ash, hackberry, American elm, red mulberry, and slippery elm also occur in the floodplain forest. Adjacent to the major river channel and in areas where the forests are limited to a narrow strip along the river bank (e.g., Fort Farm Island and Mormon Island), low shrub islands and vegetated sandbars predominate. Peach-leaf willow, sandbar willow, and indigo bush are the dominant shrub species, whereas lovegrass, nutsedge, barnyard grass, cocklebur, and scattered willow and cottonwood seedlings characterize the vegetation on the low shrub islands and recently exposed sandbars.
Some lowland grasslands have been maintained for grazing and hay production but much of the native lowland prairie on the river terraces adjacent to the floodplain has been converted to cropland. Mid- to tall-grasses such as big bluestem, side-oats grama, western wheatgrass, sand dropseed, Indian grass, switchgrass, and bluegrass characterize the vegetation on the lowland prairie (Hopkins 1951). Less numerous grasses and sedges include needle-leaf sedge, plains muhly, Canada wild rye, and small panic grass. Western snowberry, poison ivy, wild rose, black-eyed susan, American germander, fringed loosestrife, and blazing star are conspicuous forbs in the lowland prairie vegetation.
The upland prairie of the river escarpment has developed on loess-derived soils. Grasses on the upland prairies tend to be much shorter than those on the lowland prairies due to a reduction in soil moisture. Although species such as big bluestem are present in both upland and lowland prairie, their stature is much reduced on the upland sites. Blue grama and buffalo grass are the dominant short-grass species on the upland prairie, whereas little bluestem, junegrass, western wheatgrass, sedges, downy brome, and six-week fescue are less prominent grasses (Hopkins 1951). Skeleton weed, cone flower, lead plant, scarlet gaura, and wild alfalfa are characteristic forbs of the upland prairies.
Mixed short- and mid- to tall-grass prairies develop along ravines and on slopes between the upland and lowland prairie vegetation types. These mixed prairies are composed of species found in both upland and lowland prairies and have a stature intermediate between the two types.
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