SUSPENDED AND DISSOLVED SEDIMENT TRANSPORT DYNAMICS IN TWO AGRICULTURALLY DOMINATED WATERSHEDS, MCLEAN COUNTY, IL

Protecting drinking water resources is important to the economic, social, and environmental quality of the Midwestern United States. Increased sediment levels can lead to degraded water quality, reduced aquatic biodiversity, impeded recreational usage, and reduced reservoir volume. In this study I investigated the sediment transport dynamics in the main tributaries of two drinking water reservoirs in McLean County, Illinois. The two watersheds have similar hydrology, climate, and land-use, where over 80% of the land cover is agriculture, but there is a significant difference in watershed size and stream gradient. Using suspended sediment concentrations, nutrient concentrations, and field discharge measurements during baseflow and storm flow conditions, the suspended sediment and nutrient loads were generated for each sub-watershed over for the duration of the study period.

Because of different watershed characteristics, the sub-watersheds responded differently to precipitation events throughout the study period as seen by the varying discharge, total suspended sediment loads and nutrient loads. During baseflow conditions, Six Mile Creek (SMC) transported more total suspended sediment per drainage area than Money Creek (MC). However, storm flow transported 99.75% of the suspended sediment on SMC and 92.56% of the sediment on MC. MC had a stream baseflow and storm discharge (Q) that was twice as high as SMC and had sediment loads that were one to four times larger than SMC because of the larger sub-watershed drainage area of MC. Total suspended sediment (TSS) loads were seen with increasing Q and peaked in the springtime on both streams during storm events. The TSS-Q relationships were dynamic on both streams and impacted by sediment availability, precipitation, and in-channel processes like lateral bank migration, streambank erosion, and external soil erosion. SMC transported almost twice the amount of nitrate as nitrogen and chloride load compared to MC. Nutrient transportation occurred entirely during storm events on both streams.