GIS-BASED ANALYSIS OF AERIAL PHOTOGRAPHY TO EVALUATE THE RESPONSE OF MEANDERING RIVERS TO HUMAN MODIFICATION: EXAMPLES FROM ILLINOIS, USA

Meandering rivers are dynamic systems that exhibit complex mutual adjustments between process and form over time. Although the geomorphologic consequences of human-induced modifications to fluvial systems have been studied extensively, a critical unanswered question for meandering rivers is: How much geomorphic adjustment is the result of human modifications and how much is the result of internal dynamics of the system under relatively constant environmental conditions?

We examine the effects of human-modifications on the planform characteristics and dynamics of meandering rivers in Illinois, USA. Types of human modification include channelization (straightening) and installation of bendway weirs. To analyze changes in meandering rivers, a computer-based GIS tool has been developed that provides a continuous characterization of channel planform geometry and curvature. The tool also allows evaluation of changes in planform geometry over time. The tool fits parametric cubic splines to digitized channel-centerline data obtained from historical aerial photography, and characterizes the resulting composite curve using arc-length parameterization. The derivation of curvature values from a continuous planform function for any location and any spatial interval defined by the user overcomes the reliance either on average bend curvature values or on discrete curvature values based on digitized points, allowing enhanced characterization of planform geometry and improved analysis of planform change. We also present a numerical meander migration model that permits evaluation of the effects of channel modification and land-use change on planform dynamics. The computer-based GIS tool and model permit detailed analysis of the relative effects of human modification and natural lateral-migration processes on the dynamics of meandering rivers in intensively managed landscapes. Such analysis provides the basis for river management practices that seek to establish dynamically stable fluvial systems capable of supporting healthy, biologically diverse ecosystems.