Reconnaissance investigation of sediment distribution, erosion, and transport in the upper Deschutes River, Deschutes County, Oregon, November 1986

A preliminary investigation of sediment distribution, erosion, and transport in the upper Deschutes River was conducted in response to the perceived problem that brown trout spawning gravels were being rendered unusable due to an accumulation of fine-grained sediment. Contrary to the contention that channel gravels are being covered by fine-grained sediment, examination of the river channel at several sites indicates that bed sediment is generally a thin veneer ( < 0.6 ft) over local bedrock. This thin sediment veneer and numerous streambed exposures of local bedrock suggest that bed material commonly is redistributed and that most fine-grained sediments probably have a short residence time and are transported through the study reach. The Shields criterion for bedload transport suggests that maximum discharges for the Deschutes River upstream from Pringle Falls subsequent to the operation of Wickiup Dam could transport bed material about 20% layer (up to about 17 mm) than pre-closure maximum discharges. Minimum discharges subsequent to dam closure, however, appeared capable of transporting only particles about one-half as large (up to about 6 mm) as pre-closure minimum discharges. Operation of Wickiup Dam appears to have had little effect of the size of transported bedload in the Deschutes River near Benham Falls. Bank erosion, averaging 0.2 to 0.4 ft/year, appears to have started subsequent to the operation of Wickiup Dam and may have resulted from changes in the timing of high and low stages. Most of the eroded material is fine grained ( < 2 mm). Shields ' bedload transport criteria suggest that this sediment should move through the upper Deschutes River and not cause appreciable sedimentation on the channel bed. Substantial alteration of river stage following dam closure has resulted in subaerial exposure of channel bar gravels during the brown trout spawning season, which lasts from October through December in the upper Deschutes basin. This factor, combined with a lack of gravel in the deeper part of the channel, appears to be a major contributor to reduction of the brown trout spawning habitat in the upper Deschutes River.