Effects of channel bed disturbance on algal biomass in a mountain stream

This paper examines the relationship between disturbance intensity and the rate of recovery for benthic algae after the peak in snowmelt runoff in a subalpine stream. Disturbance intensity was assessed from the spatial distribution of shear stress modeled with the Multi-Dimensional Surface Water Modeling System developed by the US Geological Survey. The input data for the model (channel geometry, water surface elevation, and grain size distribution) were obtained from field surveys conducted during the summers of 2004 and 2005 in three relatively undisturbed alluvial reaches of the Williams Fork River, Colorado. The channel width of the 3 reaches varies from 10-22m, slopes vary from 0.004 to 0.015, and the median grain size of the bed surface varies from 40-85mm. Spatial distributions of shear stress were modeled in each of the study reaches for a series of relatively low flows ranging from 20-70% of bankfull. The model results indicate that peak flows in 2004 mobilized no more than about 10% of the channel bed at the two sites with lowest gradient and smallest bed material; in contrast, 24% of the channel bed at the site with the highest gradient and coarsest bed material was presumably under partial transport. Measurements of algal biomass taken at the three study sites after the peak in snowmelt runoff show that chlorophyll a levels reached values comparable to other pristine streams in Colorado. The correlation between benthic response and flow conditions was not strong at any of the sites in 2004, presumably because peak flows were far below average, resulting in little bed disturbance. Peak flows in 2005 were much higher, however, and it was evident that most of the bed was mobilized at each of the study sites. The contrast in flow and sediment-transport intensity in 2004 and 2005 provides a unique opportunity to assess the importance of disturbance on the ecology of a subalpine stream system.