Persistence in the Size Distribution of Surficial Bed Material During an Extreme Snowmelt Flood

Abstract

Sagehen Creek is a small, gravel bed stream in the Sierra Nevada of California. The reach studied has a single‐thread, meandering channel with alluvial bed and banks. Surficial bed material sampled throughout a 160‐m‐long reach has a median diameter of 58 mm. The underlying subsurface bed material is appreciably finer and has a median diameter of 30 mm. The bank‐full discharge is ∼2.0 cubic m3/s, and has been equaled or exceeded on an average of 12.1 days per year during the period of continuous gage operation, water years 1953–1983. Snowmelt runoff in the spring of 1983 was the longest period of sustained, high flows that has occurred in the past 30 years. Bed load transport rates were sampled daily at the peak of the snowmelt flood, May 30 to June 12, 1983. Whole‐channel bedload transport rates varied between 0.012 and 0.10 kg/s. An estimated 5800 kg of material larger than 45 mm was transported past the sampling cross section between May 30 and June 12, 1983. These measurements show that a significant quantity of material involving a majority of the particle sizes present in the streambed surface was transported during the period of sustained, large discharges. The size distribution of surficial bed material immediately upstream of the bedload sampling cross section was measured on June 6 near the flood peak and again on July 12 after the flood had subsided. In spite of large differences in discharge and bedload transport the size distributions of particle sizes in the surficial bed material were virtually identical. The relatively coarser layer of surficial bed material that is present in Sagehen Creek during small discharges also was in place and unchanged during a sustained period of very large discharges. Bedload sampling and measurements of tracer‐particle movement indicate that only a few particles, though involving nearly all available sizes, were entrained at any instant by even the peak flood flows.