Bed Mobility on the Deschutes River,Oregon: Tracer Gravel Results

Tracers are marked particles introduced into streams to obtain information on the movement of sediment in rivers. On the lower Deschutes River, Oregon, the threshold for bed mobility has been of interest in the relicensing of hydroelectric dams, with cross sections plotted from historical US Geological survey gauging records having shown changes at flows as low as 150 to 200 m³ s-¹ and general mobility at approximately 250 m³ s-¹. To obtain direct, empirical evidence of bed mobility, we deployed exotic gravel tracers (quartz particles) on the reach below the dam over four runoff seasons in which peak flows ranged from 146 to 192 m³ s-¹, close to entrainment threshold indicated by the cross section changes. The observed tracer movements indicated the bed experienced partial mobility (with 0-37 % of particles moving). We found no unique relation between grain diameter and critical flow strength (e.g. specific stream power), suggesting that overall bed texture and structure were more important controls on mobility than individual grain sizes.