UNDER THE INFLUENCE OF LARGE WOODY DEBRIS: A SURVEY OF THE LA CROSSE RIVER IN THE UPPER MIDWEST DRIFTLESS AREA

Streams are dynamic environments driven by the force of gravity and shaped by local climate, geology, and vegetation. Large woody debris (LWD) have an important influence on stream processes. The main influence of LWD on these systems is a resistance to flow; this added roughness induces channel adjustments, making LWD a significant part of the structure and function within a stream, which in turn have important ecological functions. No matter the functional significance of LWD, streams have been heavily managed by humankind, often involving the removal of debris to improve flow. Increases in development intensity in the lands around streams alter sediment and nutrient delivery to waterways, potentially destabilizing streams and their associated ecosystems. Recent studies have highlighted the significance of LWD in mountain streams, particularly in the Pacific Northwest of the United States and Canada. However, there has been little research on the influence of LWD on streams in the Upper Midwest. This study investigates a stream (the La Crosse River) in southwestern Wisconsin's Driftless Area. This area remained untouched by glaciers during the Last Glacial Maximum, but outwash from melting glaciers was deposited here, making the main bed material coarse sand. Analysis of the study area involved the use of remotely sensed imagery, utilizing change detection techniques to visualize the local riparian zone over the past 20 years. Stream survey methods have been utilized to assess the influence of LWD through measurements of stream (cross-sections) and LWD (total counts and volume calculation) characteristics. Preliminary results indicate that in-stream LWD does affect stream velocity in this location.