A study was made of the distribution of the fallout radionuclide Cs-137 on the surface and in the stream channel of an alpine watershed in the Colorado Front Range. Cs-137 activities of the surface (mean = 266 nCi/m2) were considerably higher than at Fort Collins, Colorado. The effects of snow-accumulation, soil-vegetation complexes and the hydrologic surface were studied in relation to Cs-137 activity. Snow accumulation areas contained significantly more Cs-137 than snow-free areas. Alpine Bog contained significantly more Cs-137 than did Alpine Turf and Alpine Meadow soils. Micro-channels and micro-depressions had significantly higher levels of Cs-137 activity than areas characterized as surface runoff. Concentrations of Cs-137 in stream bottom sediments and stream vegetation decrease exponentially downstream with distance from a permanent snowfield at the headwaters. There is some evidence that Cs-137 in sediments is accumulating in a marshy area where the stream flows across the moraine of a former mountain glacier. Cs-137 activities of the sediments were not highly related to percent silt+clay. Evidence did not support the hypothesis that most Cs-137 would concentrate in the silt+clay as it moved downstream. Significant quantities of Cs-137 were not found in stream water or the 1967 or 1968 snowpack. Measurable Cs-137 activities were found in litter and inorganic materials in and about the permanent snowfield, and in moss on rocks in the stream channel.

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