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Data in sufficient detail to support a qualitative assessment of the geomorphic response of the Rio Grande to the construction of main-stem dams are available for two reaches in New Mexico: below Elephant Butte Dam and below Cochiti Dam. Documentation of channel adjustments below Elephant Butte Dam during 1917-32 provides a set of interpretive keys to support a detailed analysis of the response of the Rio Grande to the construction of Cochiti Dam in 1973. A comparison of cross-section, planform, and profile data develops a time-sequenced picture of geomorphic change during 1970-80 in the 50-mi (80 km) reach of the Rio Grande below Cochiti. When correlated with the hydrologic records, hydraulic data, and the history of engineering activity in the reach this analysis provid s a qualitative assessment, in terms of trends, of the response of a semiarid zone alluvial river to dam construction.
Regulation of discharge and alternation of sediment load following closure of Cochiti Dam required major adjustments in the alluvial system below the dam. In the upper reaches, gravel armor and base-level control by the volume and size of sediment in arroyo and tributary deltas restricted change in the vertical dimension. Correlative adjustment in river planform was strongly influenced by channelization activities. In the lower reaches, adjustment has been through significant shifts in longitudinal profile with relatively little change in planform. Degradation was limited by gravel armor and arroyo control in the upper reaches, but became the dominant process in the middle reaches. In the lower reaches, an initial period of degradation was followed by aggradation as the heavy sediment load derived from upstream scour encountered a slightly reduced gradient and depletion of main channel flow by successive irrigation diversions. Available hydrologic data also permit evaluation of the impact of an extended period of regulated low flow and a sustained period of high flow on system stability.
A qualitative analysis, alone, yields meaningful results, and also supports a more precise assessment of river response as well as the predictive capability that can be derived from physical-process computer modeling.
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