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

Targeting of erosion and pollution control programs is much more effective if the time for fine particles to be transported through a watershed, the t ravel 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 pre sent a novel technique for tracing suspended sediment in a mountain stream using fallout radionuclides sorbed to sediment. Atmospherically-delivered B e-7, Pb-210, and Cs-137 accumulate in the snowpack, are released with its m elting and sorb to fine particulates, a portion of which are carried downsl ope into stream channels. The half-life of cosmogenic Be-7 is short (53.4 d ays), thus, sediment residing on the stream bed should contain little of th e radionuclide. The different signatures of newly delivered sediment from t he landscape with its Be-7 tag and older untagged sediment from the channel is the basis for the tracing. The total flux of such radionuclides, compar ed 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, a re transported downstream through the drainage in one or more steps having lengths of tens of kilometers. Length of the step decreases from about 60 k m 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 ol der 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 hydrogr aph at the lowest elevation location. Rates of erosion of fine sediment cal culated from the flux of radionuclides average 0.0023 cm/year. The long dis tance transport of fine particles suggests that delivery through the Gold F ork drainage to the basin outlet is fairly rapid once particles reach the c hannel and perhaps is also rapid in similar and smaller basins. (C) 1999 El sevier Science B.V. All rights reserved.