The use of tracers to study sediment sources in three streams in northeastern Oregon

This pilot study used sediment tracers to identify general source areas of channel bottom sediment within three tributaries of the Umatilla and upper Grande Ronde basins in northeastern Oregon. Land use in each stream was dom inated by agriculture, logging, or grazing. The nuclear bomb-derived radion uclide Cs-137, carbon, and nitrogen were used as tracers to fingerprint sed iment sources. Sediment was collected from the stream bottom inside the act ive channels and compared to samples from the surface horizon and channel b anks. Samples were processed to separate the <63 mu m fraction and characte rized on the basis of tracer concentrations. A simple mixing model was used to estimate the relative portion of channel bottom sediment derived from t he surface horizon and channel banks. Calculations from the Cs-137 tracer i ndicated that channel banks accounted for 56%, 74%, and 93% of the bottom s ediment in the three study drainages, although these figures have a high ma rgin of error. Cs-137 proved unexpectedly useful in the identification of a ctively eroding alluvial deposits deposited since the mid-1950s in one stud y area, likely resulting from the floods of 1964 and 1965.