Fine Sediment Dynamics in Coarse-Grained Streams: Implications for Biological Productivity in Urbanized Western Streams

Abstract

Urbanization of previously undeveloped or agricultural lands can lead to significant changes in watershed discharge and sediment yields. Increased fine sediment loads can have deleterious effects on instream biological productivity in gravel- and cobble-bed streams since both periphyton and macroinvertebrates are adversely affected by fine sediment deposition. Previous studies have shown that flushing of fine sediment to appreciable depths requires mobilization of the bed material. In the 15-Mile Reach of the Colorado River, although most of the annual sediment load in the river is transported during the snowmelt runoff period, summer storms and resulting runoff have a major impact on the spatial and temporal dynamics of in-channel fine sediment (mud) deposition and erosion that strongly influence the biotic assemblages in the river and their productivity. Mud deposition occurs where shear stress is less than 1.4 N/m 2 . Remobilization occurs when the shear stress threshold for the mud is exceeded, but these shear stresses are much lower than those required to mobilize the underlying gravel and cobble bed material. Results from 2-D hydrodynamic modeling indicate that the amount of fine-sediment-free area in riffles and runs can be predicted for a wide range of flows. Reservoir releases in the late summer period following thunderstorm events can be used to remove fine sediment without mobilizing the bed material, and can, therefore, be used to improve biological productivity.