Quantification and prediction of lateral channel adjustments downstream from Cochiti Dam, Rio Grande, New Mexico

Cochiti Dam, completed in 1973 on the middle Rio Grande in north-central New Mexico, was designed for sediment detention and flood control and traps virtually all of the sediment entering the Cochiti reach. Few large alluvial rivers in the southwestern U.S. have been studied and documented as well as this 45-km long section of the Rio Grande. Hydraulic, topographic, photogrammetric and sediment data collection efforts conducted by numerous federal and state agencies have tracked changes in the river since 1895. The primary objective of this study was to develop a method of modeling the lateral movement of the Cochiti reach based on comprehensive quantification of channel morphology and sediment and hydrologic inputs to the reach between 1918 and 1992. Lateral channel adjustments play an integral role in the dynamics of river ecosystems as well as posing potential threats to adjacent human developments.; The lateral movement rates and level of lateral stability of the Cochiti Reach were quantified using digitized aerial photos of the active channel over 74 years (1918–1992). Indices of lateral movement and stability showed that the channel narrowed and moved toward a more stable state as the peak discharges decreased prior to and following construction of the dam. Lateral movement rates declined following 1918 as the channel shifted from a multi-thread to a more single-thread pattern. Following construction of the dam, the downstream riverbed coarsened from sand to gravel size sediment and degraded up to 2 meters and the channel sinuosity increased up to 9%.; Lateral migration and width change rates of the Cochiti Reach from 1918 to 1985 were estimated using three models. Prediction of the 1985–1992 lateral movements validated the models. Width changes were estimated (r 2-values up to 0.98) using an exponential function based on deviation from an assumed equilibrium width. Multiple regression analysis produced equations that explained up to 65% of the variability in migration rates. The regression analysis highlighted significant associations between lateral movement rates and flow and planform parameters and between width change rates and active channel width. Migration rates increased with increasing flow energy, sinuosity and total channel width.