Determining the times and distances of particle transit in a mountain stream using fallout radionuclides

Abstract

Targeting of erosion and pollution control programs is much more effective if the time for fine particles to be transported through a watershed, the travel distance, the proportions of old and new sediment in suspension, and the rate of erosion of the landscape can be estimated. In this paper we present a novel technique for tracing suspended sediment in a mountain stream using fallout radionuclides sorbed to sediment. Atmospherically-delivered 7 Be , 210 Pb , and 137 Cs accumulate in the snowpack, are released with its melting and sorb to fine particulates, a portion of which are carried downslope into stream channels. The half-life of cosmogenic 7 Be is short (53.4 days), thus, sediment residing on the stream bed should contain little of the radionuclide. The different signatures of newly delivered sediment from the landscape with its 7 Be tag and older untagged sediment from the channel is the basis for the tracing. The total flux of such radionuclides, compared to the inventory in the soil, permits estimates of the rates of erosion of the landscape. Fine suspended particulates in the Gold Fork River, ID, are transported downstream through the drainage in one or more steps having lengths of tens of kilometers. Length of the step decreases from about 60 km near the peak of the hydrograph to about 12 km near baseflow. The percent of sediment in suspension that is `new' (i.e., recently delivered from the landscape) ranges from 96 to 12%. The remaining sediment is resuspended older channel sediment. Residence times for particulates range from 1.6 days, early in the hydrograph at the upper site, to 103 days late in the hydrograph at the lowest elevation location. Rates of erosion of fine sediment calculated from the flux of radionuclides average 0.0023 cm/year. The long distance transport of fine particles suggests that delivery through the Gold Fork drainage to the basin outlet is fairly rapid once particles reach the channel and perhaps is also rapid in similar and smaller basins.