The lateral migration rates of alluvial rivers are affected by changes in water and sediment regimes. The Rio Grande downstream from Cochiti Dam exhibits spatial and temporal variability in lateral movement rates documented since 1918. A tremendous database exists that documents planform, bed material size, channel geometry, and water and sediment regimes. A statistical analysis reveals that migration rates primarily decreased with decreasing flow energy (R 2 N 0.50, p b0.0001). The addition of a second parameter describing total channel width increased the explained variance to N 60%. The findings show that lateral movement increases with increasing flow energy and with degree of braiding. D

The Cochiti reach of the Rio Grande served as a case study to test the hypothesis that the lateral mobility of an alluvial river decreases as the river approaches equilibrium. The lateral mobility of the river was measured using a geographic information system from digitized aerial photographs of the nonvegetated active channel between 1918 and 2001. Reach-averaged lateral mobility was quantified in terms of width change, lateral migration, and total lateral movement. By 2001, the width of the Cochiti Reach was close to the expected equilibrium width indicating that the river had adjusted to the incoming water and sediment load. An exponential equation based on deviation from equilibrium width described 95-96% of the variance in channel width, 78-90% of variance in migration rates, and 92% of the variance in total lateral movement between 1918 and 1992. For validation of the model, the 2001 width and migration rates were predicted with errors as low as 19 and 8%, respectively. The exponential width model was also applied to four other rivers that exhibited narrowing trends following dam construction and explained 82-89% of the variance in width change on those rivers.

The impact of construction of dams and reservoirs on alluvial rivers extends both upstream and downstream of the dam. Downstream of dams, both the water and sediment supplies can be altered leading to adjustments in the river channel geometry and ensuing changes in riparian and aquatic habitats. The wealth of pre and post-regulation data on the Middle Rio Grande, New Mexico, provides an excellent case study of river regulation, channel adjustments, and restoration efforts. Cochiti Dam was constructed on the main stem of the Rio Grande in 1973 for flood control and sediment retention. Prior to dam construction, the Rio Grande was a wide, sandy braided river. Following dam construction, the channel bed degraded and coarsened to gravel size, and the planform shifted to a more meandering pattern. Ecological implications of the geomorphic changes include detachment of the river from the floodplain, reduced recruitment of riparian cottonwoods, encroachment of non-native saltcedar and Russian olive into the floodplain and degraded aquatic habitat for the Rio Grande silvery minnow. Recent restoration strategies include removal of non-native riparian vegetation, mechanical lowering of floodplain areas, and channel widening.