Short term sediment accumulation rates reveal seasonal time lags between sediment delivery and deposition in an oxbow lake

Authors: Wren, Daniel; Taylor, Jason; Rigby Jr, James; Locke, Martin; Witthaus, Lindsey

Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2019
Publication Date: 5/23/2019
Publication URL: https://handle.nal.usda.gov/10113/6449218
Citation: Wren, D.G., Taylor, J.M., Rigby Jr, J.R., Locke, M.A., Yasarer, L.M. 2019. Short term sediment accumulation rates reveal seasonal time lags between sediment delivery and deposition in an oxbow lake. Agriculture, Ecosystems and Environment. 281:92-99.

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 eroded during runoff events is deposited in lakes, and the rate at which sediment accumulates is related to the rate of erosion from the surrounding land. Knowing the amount by which management practices reduce erosion rates can help to justify expenditures by quantifying the effectiveness of the practices in comparison to ancient erosion rates. Typical techniques for measuring sediment accumulation cannot resolve rates more recent that approximately 5 years. Sediment traps, containers that sit on the bottom of a lake and collect particles that settle out of suspension, can allow for multiple rate measurements per year, which allows for measuring very recent sedimentation rates and connecting the rates to recent changes in a watershed. In the present study, sediment traps were used to measure sedimentation rates in Beasley Lake, MS, for three years (2016-2018). It was found that there was a delay between high amounts of rainfall, which led increased amounts of soil entering the lake, and the eventual settling of the particles in the sediment traps. The delay means that soil particles only settled during summer and fall months, when temperatures were higher and there was more production of algae, which likely caused particles to clump together and settle. An implication of this result is that it may not be possible to use the method to resolve changes in watershed erosion caused by specific short-term events like tillage or an intense storm. The delay in deposition means that it will only be possible to discern the integrated effects of episodic events that occur in the winter or early spring and that particles entering the lake with winter runoff may not be trapped in the lake, since they may remain in suspension and pass through the lake.

Technical Abstract: Recent sedimentation rates are useful for quantifying how changes in a watershed affect soil erosion; however, typical geochronological methods for dating sediments are limited in temporal resolution, particularly for newly deposited sediments. Methods capable of measuring the effectiveness of erosion control practices are needed, for example, by the Conservation Effects Assessment Project (CEAP), which aims to quantify the effectiveness of conservation practices in agricultural watersheds. We used sediment traps to measure short-term sediment accumulation rates in Beasley Lake, Mississippi, a natural oxbow lake whose watershed has a mix of agricultural and forested land. Precipitation data from a local Soil Climate Analysis Network (SCAN) site and water quality measurements from Beasley Lake were used to explain intra-annual patterns in sediment deposition. We found that sediment accumulation in the traps was highest in the summer months of July and August, while the highest rate of runoff, indicated by increased precipitation and total suspended solids, occurred in late winter and early spring (February-April). The delay between fine sediments entering the lake and deposition in the traps prevented the detection of changes in watershed erosion within seasons; however, sediment traps were shown to be useful for inferring changes in watershed erosion rates on annual timescales. Our results indicate that the trapping efficiency of Beasley Lake may not be as high as expected due to the potential for sediment particles to exit the lake through its outflow during winter and spring runoff before particles settled out of suspension. The rate and timing of sediment accumulation in Beasley lake was highly seasonal and occurred when water temperature and pH increased, potentially through interactions with algal blooms. Our findings on the delay between increases in suspended sediment concentration and deposition of particles in Beasley Lake contribute new understanding of the interactions between suspended sediments, algal biomass, and water chemistry in a natural oxbow lake and provide support for using sediment traps to measure intra-annual variability in sedimentation rates in oxbow lakes.