Sediment deposition and transport patterns following a reservoir sediment release

Abstract

A reservoir sediment release on the North Fork Poudre River supplied ∼7000 m3 of silt‐ to pebble‐sized sediment to an originally boulder bed channel. Deposition along the 12 km of channel downstream from the reservoir occurred primarily in pools. During the subsequent snowmelt hydrograph, sediment was progressively scoured from the upstream and then the downstream pools. Initial sediment reworking in the pools created a deep, narrow thalweg scoured to the original pool bed, with additional sediment deposition in lateral eddies. Continued reworking reduced but did not completely remove these eddy deposits. The channel became supply‐limited with respect to finer grain‐sized fractions (clay to medium sand) first at upstream and then at downstream sites and eventually became supply‐limited with respect to coarser grain‐sized fractions (coarse sand to pebbles). Bedload transport rates at a site were strongly linked to the depletion of sediment stored in upstream pools. Magnitude, duration, and sequence of flows were all important controls on bedload transport and return of the channel to its prerelease state.