Determination of lake sediment accumulation rates in an agricultural watershed using lead-210 and cesium-137

Authors: Wren, Daniel; Rigby Jr, James; DAVIDSON, GREGG - University Of Mississippi; Locke, Martin

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/8/2015
Publication Date: 3/10/2016
Publication URL: https://handle.nal.usda.gov/10113/62698
Citation: Wren, D.G., Rigby Jr, J.R., Davidson, G.R., Locke, M.A. 2016. Determination of lake sediment accumulation rates in an agricultural watershed using lead-210 and cesium-137. Journal of Soil and Water Conservation. 71(2):137-147; doi: 10.2489/jswc.71.2.137.

Interpretive Summary: Sediments stored in lakes represent a valuable archive that can be used to reveal the erosion history of watersheds. A portion of the soil that is moved down gradient during runoff events is deposited in lakes, and the rate at which sediment accumulates should be proportional to the rate of erosion from the surrounding land. The ability to calculate the rate at which this sediment has accumulated over discrete time intervals in the past opens several opportunities for improved understanding and management of watershed processes. Examples include quantifying changes in erosion and deposition rates caused by anthropogenic or natural shifts in land use or by climate change. Future efforts to conserve soil resources can be better justified if post-clearing increases in erosion rates can be quantified rather than simply inferred. Knowing the amount by which management practices reduce erosion rates can also help to justify expenditures by quantifying the effectiveness of the practices in comparison to ancient erosion rates. In the present study, sediments from Beasley Lake, MS, were dated using Pb-210 decay rates and bomb-pulse derived Cs-137 with the goal of relating trends in sedimentation rate to reductions in erosion due to management practices in specific parts of the watershed. The approach that was used did not require any knowledge of when changes were made in the watershed, making it useful for future studies in different lakes where there are seldom records of land use change. Reductions in sediment accumulation rate of about 75% showed that the steps take to reduce erosion in the Beasley Lake watershed have been effective; however, these steps also reduced the size of the particles entering lake, allowing the particles to be evenly spread over the bottom of the lake. This is a natural consequence of reducing erosion, but it prevented us from linking local land management practices with erosion rates in specific parts of the lake.

Technical Abstract: Quantifying the impact of erosion control practices in watersheds remains a difficult problem. It is rare for the application of management practices to be followed by measurements that can indicate the effectiveness of practices. The expense and time associated with erosion monitoring, along with the need to have data from before the application of BMPs, combine to prevent measurements that could result in a better understanding of how management practices impact watersheds. The accumulation rates of sediments stored in lakes can be used as an archive of past soil erosion rates in the surrounding watershed that can indicate the effectiveness of watershed management practices. This approach was applied to Beasley Lake, Mississippi, an oxbow lake in the Mississippi alluvial floodplain, to assess the impact of land use in different areas of the watershed on local sediment accumulation rates. Three new sediment cores, collected in 2011, and one core from a previous study were dated using 210Pb and 137Cs activity to arrive at estimates of sediment accumulation rate before and after large scale implementation of management practices in the Beasley Lake watershed. These data complement an earlier study, where a single core sample was used to infer the impact of management practices in the watershed. The new data confirm the findings of reduced sedimentation rates from the previous study; however, already low recent sedimentation rates around the time of implementation of additional conservation practices exceeded the resolving power of the method. Rates in all cores showed continuous decline in sedimentation rates with reduction of at least 74% with the most recently resolved rates around 0.5 cm/yr.