The activity of fallout radionuclides ( 7 Be, 137 Cs, and 210 Pb) was measured on upland and fl oodplain soils and on suspended sediments to quantify sources of fi ne sediment and to estimate sediment transport distances in stream channels in the Yellowstone River basin. Samples were collected seven times during snowmelt and runoff at nine locations from the headwaters of Soda Butte Creek to Billings, Montana, a 423-km-long reach of channel. The inventory of radionuclides in soil increases with precipitation and is highest in the headwaters. The activity of radionuclides in suspended sediment decreases downstream, and more activity is observed earlier than later in the fl ood hydrograph.The radionuclide activity of sediment derived from erosion of upland soils differs from that derived from bank erosion. Fine suspended sediment has an intermediate radionuclide signature that is quantifi ed in terms of the relative contribution of these two sources of fi ne sediment. At sites high in the drainage, soils contribute 50% to the suspended load and this value decreases to 11%-26% downstream. Fine sediment transport distances were calculated from the exponential decrease in radionuclide concentration below a point source. Transport distances increase from a few kilometers in the headwaters to hundreds of kilometers downstream. These estimates are consistent with transport distances estimated from the settling velocity of the particles and from the distribution of mine tailings downstream from a dam failure. This study of a large watershed confi rms earlier results from smaller basins and suggests that transport distances increase with basin size.

A laboratory experiment was conducted to determine the effect of tubificid worms on the flux of zinc into lake sediments. Forty-six cores of Lake Erie sediment, with and without (control) tubificid worm populations, were exposed to aquarium water with a zinc concentration of about 5 mg l-' for 139 days. Pore water and exchangeable particulate zinc concentrations in the top 12 cm of sediment were periodically determined in pairs of cores -one with worms and one without worms -at 1 cm depth increments. After 139 days, pore water zinc concentrations in sediments with and without worms were nearly identical in the O-l cm interval (4.1 and 4.3 mg I-' respectively), but were significantly greater in the sediments with worms in the l-2 cm (4.4 vs. 0.3 mg '-') and the 2-3 cm (1.3 vs. 0.3 mg l-') intervals. Exchangeable particulate zinc concentrations in the O-l, 1-2, and 2-3 cm intervals in sediments with worms were 612.3, 750.7, and 191.5 pg g-' dry sediment respectively, whereas in sediments without worms, concentrations were 375.4, 5.9, and 3.2 pg g-' dry sediment. The increased flux of zinc into tubificid-inhabited sediments was caused by the 'conveyor belt' feeding activity of the worms, which continuously exposed sedimentary particles to the overlying water. Movement of zinc into sediments with worms was dominated by adsorption and by particle movement, whereas movement of zinc into control sediments was by adsorption at the sediment-water interface and diffusion. The increased concentration of zinc in tubificid-inhabited sediments has important implications with respect to the trophic transfer of zinc through the aquatic food chain.

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