EVALUATING STREAMBANK AND IN-STREAM EROSION AS ACONTRIBUTORS TO WATER QUALITY IN TWO ILLINOIS AGRICULTURAL WATERSHEDS

Author

Chibueze Chigozie Iguegbe, Southern Illinois University CarbondaleFollow

Date of Award

12-1-2025

Degree Name

Master of Science

Department

Forestry

First Advisor

Schoonover, Jon

Abstract

This study investigates its contribution to sediment and phosphorus (P) export in two agriculturally dominated watersheds in south central Illinois—Lost Creek and East Fork Shoal Creek (EFS). Using cross-section surveys, and erosion pin measurements over a two-year monitoring period, we quantified bank erosion rates, sediment yields, and associated phosphorus loads. Hydrologic data from the Illinois State Water Survey (ISWS), alongside local precipitation and soil data, were utilized in the interpretation of erosion dynamics in the study watersheds. Key findings from this study show spatial and temporal variability in erosion rates, which is driven by seasonal flow events, riparian vegetation type, and local geomorphology. This study also sheds light on riparian buffer vegetation influence on streambank P retention, as streambanks mainly bordered by riparian forest vegetation had lower P concentration along the vertical profile compared to those bordered by grassed vegetation. Also, P concentration was found to be higher in streambanks adjacent to agricultural land than those bordered by riparian forest. This points to the lasting impact of past fertilizer use and the influence of surrounding land use on how phosphorus builds up and moves through streambank soils. These findings show how hydrology, vegetation, and land use influence erosion and nutrient dynamics in agricultural watersheds. The research supports targeted management strategies, especially riparian forest restoration and improving natural water flow regulation, to reduce erosion and improve water quality.Over a 2-year period, this study quantified the watershed-scale contributions of streambank erosion to total sediment and phosphorus (P) exports using both erosion pin and cross-section data, benchmarked against estimates from the Illinois State Water Survey (ISWS). Results show that streambank erosion in East Fork Shoal Creek (EFS) accounted for 32.8% of watershed sediment and 5.3% of P. In contrast, streambank erosion within Lost Creek contributed 22.3% of sediment and only 0.7% of P. As such, findings from this study suggest that while these watersheds are significant sources of sediment, the same is not true for phosphorus. Therefore, management efforts should focus more on investigating and controlling phosphorus loss from other non-point sources, such as agricultural runoff within these watersheds. Despite certain analytical limitations in the study, a better understanding of streambank sediment and phosphorus contributions will prove beneficial for natural resource managers and policymakers when making recommendations for effective conservation practices aimed at reducing sediment and phosphorus loading to waterbodies, thereby enhancing water quality in Illinois’ agricultural landscapes. This study is among the first to investigate streambank sediment and phosphorus loading at the watershed scale within the state of Illinois.