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 dominated by agriculture, logging, or grazing. The nuclear bomb-derived radionuclide ¹³⁷Cs, carbon, and nitrogen were used as tracers to fingerprint sediment sources. Sediment was collected from the stream bottom inside the active channels and compared to samples from the surface horizon and channel banks. Samples were processed to separate the <63 μm fraction and characterized on the basis of tracer concentrations. A simple mixing model was used to estimate the relative portion of channel bottom sediment derived from the surface horizon and channel banks. Calculations from the ¹³⁷Cs tracer indicated that channel banks accounted for 56%, 74%, and 93% of the bottom sediment in the three study drainages, although these figures have a high margin of error. ¹³⁷Cs proved unexpectedly useful in the identification of actively eroding alluvial deposits deposited since the mid-1950s in one study area, likely resulting from the floods of 1964 and 1965. [Key words: bank erosion, ¹³⁷Cs, sediment tracers, Columbia Basin, nonpoint sediment sources.]