Geomorphology and endangered fish habitats of the upper Colorado River: 2. Linking sediment transport to habitat maintenance

Abstract

Alluvial reaches of the Colorado River near Grand Junction, Colorado, provide important habitat for the endangered Colorado squawfish. This paper examines recent changes in the geomorphology of the Colorado River and addresses the question of what can be done to improve existing fish habitats. Observations of channel change during periods of above‐average runoff from 1993 through 1995 indicate minor scour and fill of the main channel, but more significant scour and enlargement of side‐channel and backwater habitats and flushing of fine sediment from the bed. Modeled relations between discharge and dimensionless shear stress т* indicate that the threshold of bed load transport (т*=0.03) occurs at about half the bank‐full discharge; these flows are exceeded about 26 days per year on average. The bank‐full discharge produces an average т* of 0.047 and is exceeded about 8 days per year. The width and depth of the channel thus appear to be set by a discharge that produces an average boundary shear stress of ∼1.5 times greater than the critical shear stress. The effective discharge is slightly less than the bank‐full discharge, and approximately 80% of the total sediment load is carried by the highest 10% of all discharges.