Case Study: Modeling the Lateral Mobility of the Rio Grande below Cochiti Dam, New Mexico

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.