VARIATIONS IN STREAMBED GRAIN SIZE AND ORGANIC CARBON CONTENT AMONG CHANNELIZED STREAMS

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. To better understand the impacts of channelization this study characterized the differences in grain size and organic carbon content in four streams, which include one unmodified stream and three streams that have been modified approximately 30 years ago, 7 years ago, and 4 years ago. Grain size analyses indicate that channelized streams have a more coarse texture than the unmodified stream. Sand sized or smaller particles comprise over 50% of the streambed in the unmodified stream; whereas, in the channelized streams, the majority of particles are larger than sand. Of the modified streams, the one modified 7 years ago has the highest percentage of coarse particles, while the most recently modified has a lower percentage of coarse grains. An analysis of the organic carbon content suggests a complex relationship between the time since channelization and the percentage of bed material composed of carbon. In the streambeds, carbon comprised between 2 and 4 percent of the material sand sized or smaller. The streams that were modified over 7 years ago had a higher percentage of organic carbon present in the bed material than the unmodified stream and the stream modified 4 years ago. Following channelization, streams will lateral erode to readjust the channel geometry as they work towards equilibrium. In these streams, lateral erosion transports organic rich top soil and underlying till to the streambed, resulting in an increase of streambed heterogeneity. The data suggest that the banks are initially more stable, but as time passes, lateral erosion becomes more prevalent.