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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.

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