CHANNEL FORMATION AND EVOLUTION FOLLOWING DAM REMOVAL

Results of two dam removal studies in southern Wisconsin include fluvial geomorphic and biogeochemical monitoring before and after dam removal. Study sites are on low-gradient, fine to coarse-grained channels with low-head, run-of-river dams that have been present for over 100 years and have impoundments filled with fine sediment. Both dams were breached in the late summer of 2000, and subsequent channel formation in the impoundments and downstream sediment transport following removal was monitored. Both channels removed large quantities of fine sediment immediately following breaching, resulting in 3-5 cm of fine sediment deposition along the wetted channel perimeter for approximately 3-5 km downstream. At the first site, channel formation in the former impoundment was rapid and consisted of vertical erosion of the channel bed, slight channel widening, and then deposition of coarse and fine sediment on the channel margins and on the newly forming floodplain. Deposited coarse sediment was derived from previously stored sediment in the former impoundment delta. At the second site, channel formation was governed by the formation and migration of a headcut. A deep and narrow channel was formed downstream of the headcut, with negligible changes occurring in the impoundment upstream of the headcut. Sediment cores taken in the former impoundments show that one channel has not formed along the alignment that it followed before dam construction. Overall results demonstrate the progression of geomorphic changes likely to follow dam removal, and offer insight and a framework for prediction and modeling efforts.