Timing and Transport of Fine Sediment in the Colorado River

Gravel-bed reaches of the Colorado River in western Colorado and eastern Utah carry an annual average sediment load of 80-90 tons/km2/yr. Although the river is primarily gravel bed, more than 95 percent of the total sediment load is fine sediment (silt and sand) carried in suspension. This sediment satisfies the traditional definition of wash load, meaning it is not abundant in bulk samples of the bed material taken from low-lying gravel bars, but it is present in different areas of the channel bed characterized by low velocity; these areas include channel margins, secondary channels, or shallow pools, which are common to all reaches. Thus, in any given year, the suspended load carried by the Colorado River reflects varying proportions of sediment supplied from the surrounding watershed and sediment supplied from the channel bed. Analysis of suspended sediment data from USGS gauging stations in the study area indicates that peak sediment concentrations occur on the rising limb of the annual snowmelt hydrograph; the majority of this sediment is silt and clay. As snowmelt discharges increase, the amount of sand in suspension increases, reaching about 30 percent at flows equal to the threshold for bed material (gravel) transport. Sand transport continues after the peak in the hydrograph. The peak in suspended sand concentration occurs, on average, about 2 weeks after the peak in water discharge. At this point in time, suspended sediment concentrations are lower, but the supply of fines is not completely exhausted from the bed. Sediment samples obtained from traps in the riverbed indicate that fine-medium sand (0.25-0.5 mm) continues moving as bed load several weeks after the peak in the hydrograph. The sand fraction of the total load is thus not easily distinguished as wash load or bed-material load. Efforts to improve habitats for endangered fishes in the Colorado River through coordinated reservoir operations must therefore take into account the potential ecological effects of the supply and movement of fines both before and after the peak in the annual hydrograph.