SEDIMENTOLOGY OF A MODERN BRAID BAR IN THE WISCONSIN RIVER NEAR SPRING GREEN, WISCONSIN: ANALYSIS USING GROUND-PENETRATING RADAR

A ground-penetrating radar (GPR) survey was conducted over a sand braid bar in the Wisconsin River near Spring Green, Wisconsin to characterize depositional structures within a longitudinal bar. The clean (little to no clay or silt), medium to coarse grained sand in the Wisconsin River provided an ideal environment for GPR surveying, and excellent penetration depth (6m+) and resolution were achieved. A Sensors and Software PulseEkko 1000 with 250 MHz antennae was used to collect data on a 20m x 20m grid over an area of approximately 40,000 square meters. Data were also collected on a 0.5m x 1m grid over an area of 300 square meters near the upstream end of the bar to provide a detailed dataset for that area in an attempt to observe upstream accretion reflectors. Aerial photography and detailed topographic surveys were used to characterize the bar externally and to monitor bar migration. USGS gaging stations upstream and downstream of the site provide a history of discharge data that can potentially be linked to depositional patterns within the bar. Acquired radar profiles reveal the internal architecture of the bar which consists primarily of downstream, lateral, and vertical accretionary deposits. Some upstream accretion, as well as many abandoned channels and former bars which can be correlated between multiple transects were interpreted from the radar profiles. Combination and further analysis of the data will be used to determine dominant depositional processes in this reach of the Wisconsin River, and to reconstruct a depositional history of the bar. Additional topographic surveys and aerial photo shoots are in progress.