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Title: THE USE OF TRACERS TO STUDY SEDIMENT SOURCES IN THREE STREAMS IN NORTHEASTERN OREGON

Author
item NAGLE, GREGORY - CORNELL UNIV, ITHACA, NY
item Ritchie, Jerry

Submitted to: Physical Geography
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Carbon, nitrogen, and fallout Cesium-137 (Cs-137), were used as tracers to determine sediment sources from agriculture, logging, or grazing watersheds of the Umatilla and upper Grande Ronde basins in northeastern Oregon. Sediment was collected from the stream bottoms inside the active channels and compared to samples from the surface horizon and channel banks. Radioactive fallout Cs-137 proved better than carbon and nitrogen in determining sediment source. Measurements from the Cs-137 indicated that stream bank erosion accounted for 56, 74, and 93 percent in the agriculture, logging, or grazing watersheds, respectively. Cs-137 was an excellent tracer for estimating sediment sources in these watersheds; however, developing methods to control bank erosion in these watersheds will be difficult and expensive.

Technical Abstract: 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 radio-nuclide Cesium-137 (Cs-137), 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 less than 63 mm fraction and were 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 Cs-137 tracer indicated that channel banks accounted for 56, 74, and 93 percent of the bottom sediment in the three study drainages; although, these figures have a high margin of error Cs-137 proved useful in the identification of actively eroding alluvial deposits laid down since the mid 1950's in one study area, likely resulting from the floods of 1964 and 1965.