DISTRIBUTION OF ORGANIC CARBON AVAILABILITY IN MODIFIED STREAM SEDIMENTS, MACKINAW RIVER BASIN, McLean COUNTY, IL

Stream channelization, which entails reducing the sinuosity of a stream, widening, and in some cases deepening the stream channel, is a common practice in agricultural regions. Channelization efforts in central Illinois have tremendous impacts on the geomorphology, flow regime, sediment and nutrient transport, and availability of aquatic habitats both in, and adjacent to modified reaches. Denitrification is highest in organic rich materials such as in debris dams or gravel bars where carbon is available. The goal of this study is to characterize the changes in stream channels by comparing four streams that are at different stages of recovery post-channelization, and to estimate a rate at which this process occurs. Organic carbon content for the study was determined using the loss on ignition method detailed by Schulte and Hopkins (1996). An initial analysis of the results indicates that carbon availability decreases with time after channelization, and that unmodified streams have limited amounts of carbon availability. However, when comparing this to grain size data, streams that have never been modified or were modified over 30 years ago possess organic carbon at levels much higher than in recently modified streams. Recently modified streams lack fine-grain sediment, whereas the unmodified stream and the stream modified 30+ years ago, both observed in the study, have higher percentages of fine-grained material that includes most of the available solid organic carbon. In a stream modified 5 years ago, the streambed has a high percentage of organic carbon, indicating that the primary contribution is from bank soil. Overall the trend for organic carbon with respect to grain size indicates an inverse relationship, with clays and silts having the highest percentages.