Channelization Effects on the Rio Puerco Above La Ventana, New Mexico

Michael Coleman, New Mexico Environment Department
Allen Gellis, U.S. Geological Survey
Dave Love, New Mexico Bureau of Mines
Richard Hadley, U.S. Geological Survey (Retired)

A 1.1-mile reach of the Rio Puerco upstream from La Ventana was channelized between 1965 and 1966. The channelization nearly doubled the channel slope from 0.004 to 0.0074 feet/feet and decreased the incised width and sinuosity. During the next 30 years the channel responded to channelization by eroding both vertically and laterally, decreasing the gradient through exposed alluvium and along bedrock shelves, widening the channel, and increasing the sinuosity. The lower channelized reach is incised more than 50 ft deep in alluvium and widened from 20 to more than 300 ft. As much as 130 ft of lateral erosion has taken place since 1973. Power lines have had to be relocated twice and the largest meander now threatens the highway alignment. The upper channelized reach is incised 25-35 ft into sandstone and shale bedrock, but has only widened where alluvium rather than bedrock forms the walls. A waterfall and set of smaller cascades over sandstone ledges form headcuts within the bedrock reach. Based on a series of aerial photographs taken between 1973 and 1994, the headcut waterfall is advancing northward at an estimated rate of 4 ft/yr. Approximately 14.1 million cubic feet of sediment were eroded during the past 30 years or about 470,000 ft3/yr. This quantity is approximately 20 percent of the river's annual suspended sediment load near its mouth.

Following initial channelization, the Rio Puerco proceeded through stages of channel adjustment as it tried to establish a new equilibrium. The river widened and increased sinuosity, which caused bank failure and mass wasting as the channel expanded against its vertical walls. At the lower end of the bedrock reach, the channel was deflected to the east by a bedrock knob, causing an initial meander cutting into alluvium. Downstream, bank failure enhanced meander development with progressive meander wavelengths decreasing to a straight channel prior to reentering the natural channel. Meanders now have developed point bars and have begun to store sediment as well as continue to erode the outer banks to continue furnishing sediment for transport and storage.

Based on evolution of the channel during the past 30 years, the channel will continue to migrate laterally and pose a threat to the road and to other human-made structures.

Introduction
The Rio Puerco of the East in central New Mexico is one of the most sediment-laden streams on earth. Suspended sediment concentrations of more than 600,000 parts per million (60 percent) have been recorded at the river's lower end and pose a threat to agriculture and to reservoir storage downstream (Gorbach and others, 1996). The stream drains more than 7,295 square miles east of the Continental Divide to the Rio Grande from the Zuni-Acoma sub-section of the Colorado Plateau and the Nacimiento Mountains of the Southern Rocky Mountains. The stream is ephemeral over much of its length, flowing only in response to precipitation. It also loses water along its path, flowing 53 percent of the year near Guadalupe, midway along its course, but only 20 percent of the year near its mouth. The main channel and many of its tributaries are deeply incised in the landscape, being eroded from 20 to 60 feet below the level of the former valley floors.

In the 1960's an uncontrolled engineering experiment began along an incised, meandering reach of the Rio Puerco near La Ventana, New Mexico. As a result of highway construction, a 1.1-mile reach of the river was channelized into a nearly straight, incised artificial course that has continued to erode and cause problems during the next 30 years. The purpose of this article is to describe the evolution of this artificial reach and some of the consequences to the stream and to human-made structures.

Methods
Many of the changes documented for the artificial reach of the Rio Puerco near La Ventana are taken from topographic maps and aerial photographs produced by the U. S. Geological Survey, U. S. Bureau of Land Management, U. S. Forest Service, and New Mexico State Highway Department (NMSHD). Copies of the aerial photographs were obtained from the Earth Data Analysis Center at the University of New Mexico, from the New Mexico Bureau of Mines and Mineral Resources, and from the NMSHD. Engineering plans for the channelized segment were examined at the NMSHD. Photo sets pre- and post-dating the straight channel (1935, 1954, 1973, 1975, 1986, 1989, and 1994) were examined to document the sequence of impacts on the river. Co-author Richard Hadley had the foresight to take ground-based photographs of the artificial reach in 1967, 1968, and 1969. Later, he and co-author Dave Love photographed the reach in 1986. Allen Gellis and Scott Aby surveyed the affected reaches (both new and old) using a Total Station during the late fall of 1997.

Channelization History
The main access road between La Ventana and Cuba formerly traversed a series of drainage valleys to the east-northeast of the present alignment. Prior to the new highway construction and channelization activity above La Ventana, the Rio Puerco occupied a sinuous, lower gradient river channel in a relatively confined (topographically) river valley segment.

As shown in figures 1, 2 and 3, a straight, mostly north to south diversion channel segment ("ditch") was dug to bypass the natural meandering channel between 1965 and 1966. Copies of the original engineering plans, provided for our examination, show the original artificial channel was designed to incorporate a 10 foot to 20 foot bottom to the channel, defined by sloped banks having a 60 foot slope length. It is apparent that the original channel cut was not completed as designed, perhaps because it cut into resistant bedrock lithologies and a smaller, shallower channel was deemed adequate. To the north the natural channel dimensions are over 150' wide. The channel was forced to enter a 60-foot wide artificial channel mouth, which funneled down to the 20 foot wide channel after only 100 feet. A large earth-fill dam structure isolated the flow and diverted all the Rio Puerco flow into the new north-south "ditch". Within the old meander zone another earth fill berm cut off a recumbent meander loop to provide road base for the new highway. A bulldozer cut into bedrock was designed to keep any remnant stream flow away from the east side of the road embankment. At the southern end of the meander set a third earthen dam was constructed over a box culvert to enable a minor amount of local drainage to continue to access the Rio Puerco. The 5,500 foot long straight channelized segment was reintroduced to the original Rio Puerco channel at the south end of the blocked-off meandering segment.

Geomorphic Response
Geomorphic changes in the Rio Puerco channelized reach through time and space were quantified from examination of the aerial photographs and interpretations from 1997 surveys of the channelized portion.

Change in Slope
Cutting of the new straight channel and forced abandonment of the meander zone significantly steepened the local stream gradient by shortening the river's local channel length, from 13,770 feet to 8,159 feet (a difference of 5,611 feet), within the area where elevation decreases from 6600 feet a.s.l. to 6540 feet (figs. 1, 4a). The slope of the original Rio Puerco channel (as measured off existing topographic map coverage) was 0.004 vertical feet per longitudinal foot (60 feet elevation drop over a distance of 13,770 feet). The slope of the constructed channel nearly doubled to 0.0074 feet/foot (60 feet /8159 feet). Surveys of the channel in November, 1997 indicate the present slope of the upper portion of the channelized segment exceeds 0.008 ft/ft (figure 4b).

Channel width, depth and slope tends to approach an equilibrium state to transport the available discharge and sediment load. The concept can be expressed as an equation (Bull, 1991):

Stream Power (driving factors) / Resisting Power (resisting factors) = 1.0

Where driving factors are stream discharge and channel slope and resisting factors are sediment size, hydraulic roughness, and bed load. A change in any of these variables will cause an adjustment in the channel. In the case of the Rio Puerco the approximate doubling of the channel slope increased the driving factors and led to significantly accelerated stream bed incision into unconsolidated, highly erosive, river valley fill, and downward into easily eroded Cretaceous Mesa Verde Group bedrock. Mass removal of fill and bedrock is actively underway, as evidenced by headcutting, downcutting and widening of the original "ditch" into a deep and wide vertical walled canyon. All of this eroded sediment is transported downstream and exacerbates sediment loading impacts on downstream reaches, perhaps as far as the lower river where the Rio Puerco system empties into the Rio Grande.

Headcut Movement
The evolution of the channelized segment of the Rio Puerco has been documented from aerial photographs and provides a model for predicting future channel changes. The immediate response of the channel to straight channelization and increased slope was downcutting. The initial incision of the artificial channel reached layers of interbedded resistant and erodible bedrock of the Cretaceous Mesa Verde Group sediments (a sandstone of the La Ventana Tongue of the Cliff House Sandstone) and a series of small stair steps and waterfalls have been produced (figures 2, 3e, and 5). One especially impressive sandstone ledge exposed within the channelized segment has developed into a 15 foot high waterfall. Field surveys indicate that the channel has incised to a depth of 30-35 feet above this ledge, expanding to over 50 feet deep in reaches below the sandstone waterfall.

The waterfall marks the present position of the most active headcut feature in the channelized segment. Studies of air photos taken 1973-1994 provide an estimation of the rate of erosion, indicating a 70 foot+ march upstream over 21 years and a widening of the channel below the headcut (figure 5). The rate of upstream headcut advancement, measured by the position of the bedrock waterfall, is just slightly less than 4 feet per year. Below the falls the lateral erosion of the vertical channel walls is observed at approximately 12.4 feet per year (260 feet/21 years), indicated by the widening of the 30-35 foot wide channel (above the falls) to a current width of >300 feet downstream, during the period 1973-1994. It is quite plausible to predict that the falls will continue to migrate upstream, exposing underlying nonresistant shale to erosion, deepening and widening the channel, promoting downstream widening. Incision of the bed of the Rio Puerco will create a lower base level for the tributary streams.

Sinuosity
Natural river channels exhibit a tendency to develop meander forms which are proportional to the regional slope, the channel size, the range of flow stages which the channel is capable of containing, and the size of sediment materials in the bed and banks. The original Rio Puerco meanders are part of a characteristically sinuous pattern with appropriate belt width development (fig.2), which exists over the vast majority of the watershed. The straight segment is anomalous and artificial, thereby throwing unnatural effects into the process.

Following artificial entrenchment, a narrow deep channel was formed that began to meander and widen by mass wasting and in-channel flow removal of sediments (fig. 3c). The southern two-thirds of the channelized reach has widened from the original 10-20 feet to the present 40 to >300 feet. The meander length of the natural channel is considerably greater than the present reach and is a solid indication that this stream reach will continue to widen as it approaches a new equilibrium (fig. 2).

Based on changes observed in the channelized reach, the river evolves first by incision followed by channel widening, especially in the valley fill portion. These changes will follow the advancement of the headcut waterfall as it advances upstream. Similar changes in channelized streams have been observed elsewhere (Schumm and others, 1984; Simon, 1989).

Estimation of Sediment Removal: MassWasting Rate
Examination of the original Highway Department / Burnett Engineering plans for this highway and river segment indicates the designed channel change was to be a diversion ditch with a 20 feet bottom width, supporting slopes occupying a 60 foot slope width. This would have resulted in a channel cut 53 feet deep, 72 feet wide with the 20 foot bottom zone designed to transmit the flow. It is apparent that hard bedrock ground conditions prevented the construction crews from achieving the original design. The channelized reach was vertical-walled, approximately 20 feet deep and perhaps originally designed at 20 feet wide. The channel just above the waterfall has expanded to a current depth of 30-35 feet and a width of 42 feet. This is the relatively inactive upper segment of the channelized reach (afforded some protection by the bedrock waterfall), but it has deepened 10-15 feet (50-75% increase) and widened approximately 20 feet (100%). The channel dimensions below the major headcut presently range to over 300 feet wide (a lateral increase of >260 feet of bank materials has been removed, resulting in a 1,500% increase in width) and the floor depth has dropped another 15 feet below the falls. Based on these assumptions, the volume of materials eroded (to date) from this channelized reach, and the amount of projected erosion can be conservatively estimated (upper/lower segment division drawn at the waterfall: average measurements used):

(Past:) 1). Current void volume:
(upper segment:) 40'w X 32'd X 1800' lg = 2.304M ft³ plus:
(lower segment:) 150'w X 40'd X 3400' lg = 20.4M ft³

Subtotal 22.704M ft³ minus:

2). "original" channelized volume (data from NMSHTD plans): 8.6M ft³ (319,426 yd³)

Resulting estimate of erosion: ~14.1 million cubic feet of sediment were removed over the past 30 years = >470,000 ft³/yr (X 90lb/ft3 /2000lb/ton =21,150T/yr).

(Future:) Bank and streambed removal can be estimated in a similar manner. Consider an upstream advancement of similar channel dimensions as the ones developed since 1973: one mile in length, 15 feet in additional depth, 260 feet wider channel; multiplied by an estimated average density of 90 lb/ft³ (for the combined alluvial and bedrock materials):

(Projected:) 1). 15' X 260' X 5280' X 90 lb/ft³ / 2000lb/ton = 926,640 T.

Converted to cubic yardage: 2). 15' X 260' X 5280' = 20,592,000 ft³ / 27 ft³/yd³ = 762,666 yd³

Using these calculations, as the headcut advances and the upstream channel portion comes to resemble the widened and deepened portion below the falls, this river reach is capable of removing over 920,000 tons of sediment per mile of channel, as the channelized segment advances northward. This estimation represents over 20% of the 4.44 million tons average suspended sediment load the Rio Puerco delivers to the Rio Grande on an annual basis, based on data collection records from 1948 through 1996 (Gellis).

Summary
Channelization in a segment of the Rio Puerco near La Ventana in 1965-67 led to geomorphic changes in the river. Channel slope before the channelization was 0.004 feet/ft. In 1997 upper portions of the channelized reach exceeded 0.008 feet/ft. The increase in slope led to dramatic vertical and lateral changes in the river. The channel has incised to depths over 50 feet. A 15 foot high knickpoint developed in bedrock of the Mesa Verde group. The rate of knickpoint migration measured with air photos indicates movement of 4 feet per year. Below the knickpoint the lateral erosion of he channel wall is 12.4 feet per year to a current width over 300 feet.

Based on changes in width and depth of the Rio Puerco channel it is estimated that over 21,150 tons per year have been removed. As the knickpoint migrates upstream it is estimated that over 920,000 tons of sediment are produced per mile of channel. This represents 20 percent of the 4.44 million tons of suspended sediment transported annually by the Rio Puerco to the Rio Grande.

References
Bull, W.B., 1991,Geomorphic responses to climatic change: Oxford University Press, 326 pp.

Gorbach, C., Love, D., Piper, S., Davis, R., and Cross, A., 1996, Rio Puerco sedimentation and water quality study: Preliminary Findings Report: U. S. Bureau of Reclamation, Albuquerque Area Office, 101 p.

Schumm, S.A., Harvey, M.D., Watson, C.C., 1984: Incised channels: Littleton Press, Littleton, CO, 200pp.

Simon, A., 1989: A model of channel response in disturbed alluvial channels; in Earth Surface Processes 14: p11-26.

U.S. Department of the Interior
U.S. Geological Survey
This page is http://esp.cr.usgs.gov/rio_puerco/papers/channel.html
Maintained by Richard Pelltier
Last modified: 15:04:19 on 15-Mar-2006