LINKING CHANNEL EVOLUTION TO SEDIMENT TRANSPORT FOLLOWING THE REMOVAL OF THE ST. JOHN DAM, SANDUSKY RIVER, OHIO

Dam removal is probably the single most important tool for restoring habitat quality and ecosystem function to riverine systems. One of the major issues surrounding dam removals is the transport and deposition of sediment downstream following deconstruction of the dam structure. In this paper we present an extensive data set from the St. John Dam removal (Sandusky River, northern Ohio) that shows how changes in channel morphology affect sediment transport rate and thus, export of sediment to downstream reaches. Other studies have either quantitatively or qualitatively shown increased sediment loading following dam removals. During the removal of the St. John Dam, sediment loads (both suspended and bed loads) were low immediately downstream of the dam. In the months following the removal, overall transport remained low, however, sediment transport was higher upstream, in the former reservoir, than in several downstream reaches (from 75 m to 3600 m downstream), and in a control section, 13 km upstream of the dam. Downstream reaches did accumulate some fine sand, presumably from the reservoir, following the dam removal but this had a minor effect on channel geometry and slope below the dam. The most noticeable change was in the longitudinal slope of the channel in the former reservoir. Here the channel slope decreased by 30% as sand bars in the reservoir were eroded and deposited in large pools. Because of the low sediment transport rates following the removal of the St. John Dam, our study represented a paradox when compared to increased transport in other restored rivers. A modified form of the Bagnold equation was used to show that the decreased channel slope in the reservoir was consistent with the low export of sediment downstream. This analysis was then applied to other dam removals with the result that increased transport for these removals was most likely the result of increased channel slope -explaining the apparent paradox. We conclude that not all dammed rivers contain high volumes of sediments in their reservoirs and that channel slope upstream of the dam is one of the keys to predicting sediment loading following dam removal.