SYSTEM-WIDE CHANGES IN THE DISTRIBUTION OF FINE SEDIMENT IN THE COLORADO RIVER CORRIDOR OF GRAND CANYON NATIONAL PARK

The riverine ecosystem of the Colorado River between Glen Canyon Dam and Bright Angel Creek has less fine sediment on its bed, in eddies, and as channel-margin deposits in 2001 than it did prior to completion of the dam. The decrease in fine-sediment storage is documented by comparison of historical oblique photographs, analysis of historical aerial photographs, and field surveys since 1990. The synthesis of these data is the most comprehensive and systematic every conducted for the Grand Canyon ecosystem.

The magnitude of the decrease in fine-sediment deposis is uncertain. The loss of sand is probably about 25% of the area typically exposed at base flow in pre-dam photographs, but estimates range between 0 and –55%, depending on study reach and method of analysis. There is no indication that the magnitude of decrease is less in the downstream part of the study area. The cumulative loss of eddy sand is about 1 m in thickness but also varies greatly.