『Abstract
The activity of fallout radionuclides (7Be, 137Cs,
and 210Pb) was measured on upland and floodplain soils
and on suspended sediments to quantify sources of fine sediment
and to estimate sediment transport distances in stream channels
i the Yellowstone River basin. Samples were collected seven times
during snowmelt and runoff at nine locations from the head waters
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 flood 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 quantified in terms of the relative contribution of these
two sources of fine sediments. 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 confirms earlier results from smaller basins
and suggests that transport distances increase with basin size.
Keywords: sediment transport; erosion; 7Be; 210Pb;
Yellowstone National Park; fluvial geomorphology.』
Introduction
Study area
Methods
Soil sampling
Snowpack and precipitation sampling
Suspended sediment sampling
Streamflow
Gamma spectroscopy
Results
Hydrology
Snowpack and precipitation
Upland soils
Nature of the profile
Radionuclide inventory trends
Suspended sediment
Sediment size and concentration
Radionuclide activity of suspended sediment
Hysteresis in sediment and radionuclide activity
Spatial pattern to radionuclide activity and fluxes
Discussion
Sources of sediment
Relative contributions to suspended sediment
Transport distances of suspended sediment
Conclusion
Acknowledgments
References cited