Geomorphology of the Colorado River in the Grand Canyon

Sediment supplied to the Colorado River within the Grand Canyon has been sorted into distinct deposits of three grain size ranges. The major rapids are formed by boulder deposits from side-canyon tributaries. As a result of a fourfold reduction in peak discharge when Glen Canyon Dam was closed in 1963, new fan debris may increase the gradient through some of the rapids by a factor of 1.8. Cobbles and gravel, transported only during flood stages, are preferentially deposited in the wider sections of the river as bars and riffles and are, for the most part, inactive during post-dam discharges. Fine-grain (largely sandy) terraces occur throughout the canyon, especially along the banks of the large reverse eddies above and below the rapids. The lower terraces are being reworked into beach-like shores by diurnally-varying, post-dam discharges. A slight net lateral erosion of the terraces has resulted. Prior to construction of the dam, sandy bed deposits underwent scour averaging about 1 m during spring floods, balanced by deposition from tributary sources during the summer. Downstream from rapids, decreased turbulence due to lower discharges has resulted in deposition averaging 2.2 m on the bed within the upper portions of the canyon. Differences in rock types along the river determine overall channel morphology. Rocks of low resistance result in a wide valley, a meandering channel, and abundant cobble bars and sand terraces. Narrow channels with rapids and deep pools are most frequent within the sections of the canyon where Precambrian crystalline rocks dominate.