Nationally, sediment in reservoirs is a concern
as related to several important issues including water quality and reservoir
water-storage capacity. The ability to achieve meaningful decreases in sediment
loads to reservoirs requires an understanding of the relative contribution from
sediment sources within the contributing watersheds. To estimate sources of
sediment within the 2,900-km2 Perry Lake watershed of northeast
Kansas, representative samples of channel-bank sources, surface-soil sources
(cropland and grassland), and reservoir bottom sediments were collected,
analyzed, and compared. Within the watershed, a nested approach was used in
which five representative subwatersheds were selected
for sampling. The subwatersheds ranged in size from
20 to 50 km2. Samples were sieved to isolate the <63-micron-size
fraction and analyzed for particle-size distribution, nutrients (total nitrogen
and total phosphorus), carbon, 25 trace elements, and cesium-137 (137Cs).
Total nitrogen (TN), total phosphorus (TP),
total organic carbon (TOC), and 137Cs were selected as the
constituents that provided the best ability to discriminate between
channel-bank and surface-soil sources of sediment in the Perry Lake
watershed. To adjust for differences in particle-size composition, constituent
ratios and clay normalization were used. Computed ratios included the ratio of
TOC to TN, TOC to TP, and TN to TP. Constituent concentrations (TN, TP, TOC)
and activities (137Cs) were normalized by dividing by the clay
(<4-micron) content. Thus, the estimation of sediment sources involved the
use of seven indicators (three constituent ratios and the clay-normalized
values for four constituents).
Within the Perry Lake
watershed, sediment contributions from channel-bank and surface-soil sources
varied among the subwatersheds. However, for Perry Lake,
channel-bank sources were predominant indicating that these sources increased
in importance with increasing distance downstream in the watershed. In this
presentation, methods, problems, results, and possible implications are
discussed in the context of previous studies.
