PHOTOANALYTIC ASSESSMENT OF DYNAMICS IN TRIBUTARY STREAMS OF THE ILLINOIS RIVER BASIN

What is the response of streams to induced change? Diverse state, federal, and local organizations in the Illinois River basin are attempting to coordinate efforts to preserve and restore ecosystems and reduce loss of land to erosion. This critical question must be answered to assess impact of mitigation projects. Most of the streams in Illinois have been altered by people to some degree, and have experienced hydrological changes over the past century. Possible reponses to these influences include stasis, incision, channel widening, and lateral migration. We have developed methods to quantify change in stream planforms through analysis of aerial photos. In this project, we use these methods on historical aerial photography to characterize the variability of dynamic behavior in streams across the Illinois River basin. Our image database includes aerial photography obtained from the 1930's through the 1990's at approximately 10-yr intervals. One-mile-long reaches of 10 streams, each with at least several decades of monitoring data, were selected for a pilot study. The channel threads were digitized from scanned and orthorectified photos, and the threads were buffered to the thickness of the root mean square error of the rectification. The buffered threads were compared by overlaying them digitally and calculating overlapping and non-overlapping polygons. Non-overlapping polygons were considered to represent significant change and were classed as channelized or sinuous. Quantified changes were evaluated in light of stream power calculations from gauge data, geology and soils data, digital and field measurements of geomorphology, and changes in land use and land cover data. Channel evolution between sinuous reaches on consecutive images represents natural variability of stream erosion and deposition, manifested as lateral migration and downstream translation. Evolution from a channelized to sinuous condition represents an indirect response of the stream to human modification, whereas a change from sinuous to channelized marks a direct human impact on stream alignment. The response of streams to channelization is particularly important because it provides important information for evaluating the feasibility of restoration projects focusing on dechannelization of streams.