The Contribution of Streambanks to Suspended Sediment Flux

Along many of our rivers there is legacy of sediment delivered to valley bottoms by previous landuse practices. To develop frameworks for improving water quality and stream morphology, it is often important to understand and quantify the sources of sediment to channels.

We establish the contribution of streambanks to the suspended sediment flux in a variety of settings with a two-component mixing model using the distinctive radionuclide signatures of sediment eroded from streambank sediment and sediment from the rest of the landscape. 7Be and 210Pb are both delivered from the atmosphere by wet and dry fallout. 7Be has a short half-life (53-days) compared to 210Pb (30 years) and as a consequence, the activities of the radionuclides differ dramatically in the soil column. Erosion of the surface of soils yields sediment with high activities of both 7Be and 210Pb whereas erosion of the streambank provides sediment with lower activities. Measurement of the radionuclide activities of suspended stream sediments and comparison with the landscape and bank end members establishes their relative contributions to suspended sediment.

Along a 423-km-long reach of the Yellowstone River, the average percentage (over the snowmelt hydrograph) of suspended sediment from the banks increased from 50% at the upstream-most site to 89% at Billings, MT. The percentage was lower at higher flows. In Old Woman Creek, OH, 63% of the suspended sediment at high flows came from the banks; in Fish River, AL, 64%; and in Winchester Creek, OR, 45%.

Streambanks appear to contribute a larger proportion of the fine sediment flux at lower stages and lower in the basins. In basins where a significant portion of the landscape is presently used for agriculture or silviculture, the percentage of sediment from streambank erosion drops below 50%.