Assessing streambank and in-stream erosion as a source of sediment and phosphorus in two Illinois agricultural watersheds

Author

Justin Kazuki Higa, Southern Illinois University CarbondaleFollow

Date of Award

8-1-2024

Degree Name

Master of Science

Department

Forestry

First Advisor

Schoonover, Jon

Abstract

Streambank erosion is known to contribute significant amounts of sediments and nutrients to receiving waterbodies and can constitute a major proportion of nonpoint source pollution. Phosphorus, which tends to bind to sediment, can be lost through bank erosion and be released to the receiving water body. Thus, controlling sediment losses from streambank erosion could translate into a way for reducing phosphorus loss, producing a convenient reduction in two of the major pollutants specified by the Illinois Nutrient Reduction Loss Strategy (IEPA, 2023). However, the degree of contributions from streambank erosion is often unknown and contributions can vary between geographic locations. Studies in other locations within the continental United States estimate streambank erosion to account for 4% to 100% of the total watershed sediment yield and from 2.7% to 100% of the total watershed phosphorus yield, signifying the wide range and variable impact that erosion has on downstream water quality. No studies examining streambank erosion contributions to total sediment yield have been completed within the state of Illinois thus far. As such, this study provided an opportunity to explore methods used in past studies for application on small Illinois agricultural streams, as well as form initial estimates for streams in watersheds with an agriculture-dominated land use in the alluvial soils of south-central Illinois. Lost Creek and East Fork Shoal Creek, two small agricultural watersheds that are a part of the greater Kaskaskia watershed, were selected to build off existing monitoring infrastructure maintained by the Illinois State Water Survey. Initial estimates of streambank erosion over an August 2022 to August 2023 monitoring period attribute 18% of the watershed sediment yield and 2% of the watershed P yield to stream erosion for Lost Creek, while estimates for East Fork Shoal Creek attribute 26% of the watershed sediment yield and 3% of the watershed P yield to steam erosion. This study concludes that within these watersheds, streambank erosion, while a significant contributor to watershed sediment exports, do not contribute a significant portion of the watershed P export. Streambank TP concentrations were also assessed to identify any effects of land use on soil nutrient levels. Analysis of samples directly obtained from bank surfaces indicated that the proximity of agricultural land to a streambank is associated with higher TP concentrations in streambank soils, while no differences in soil TP were identified between different groups of riparian vegetation or historical land uses. Ultimately, this study finds that streambanks did not contribute significantly to watershed phosphorus exports over the study period, nor do streambanks store a significant amount of TP when compared to agricultural field sites. As such, land management should still focus on agricultural soil and nutrient loss as the main target for nutrient loss reduction goals.