Variations in Organic Particle Deposition Rate and Stream-SubsurfaceExchange due to Silt Accumulation in a Gravel Bed

The deposition of natural suspended sediments into a gravel bed was investigated in two parallel 29m-long flow-through flumes. We were particularly interested in the deposition of Particulate Organic Carbon (POC), which represents an important energy source for the benthic ecosystem. We hypothesized that, as silt accumulates in fresh beds, there should be a decline in both the stream-subsurface hydraulic exchange and the effective deposition velocity of organic particles. Stream water was pumped directly through the flumes, so that there was a continual input of natural suspended stream sediments into the channels. The two flumes were both run in uniform flow, but with different bed sediments, slopes, and roughnesses. To observe particle deposition, additional suspended matter was obtained from the stream and injected into the flumes as a pulse input. Particle concentrations were then monitored at the downstream end of each flume. Solute injection experiments were conducted similarly in order to quantify stream-subsurface exchange fluxes. These experiments were repeated over a period of several months in order to determine the effect of stream bed siltation on subsequent deposition. The rate of net particle deposition decreased over time, indicating that the cumulative deposition of fine material altered the bed surface and reduced the flux of particles to the stream bed. Leaf accumulation in the channels also appeared to have a significant effect on particle deposition.