HISTORICAL CHANGES IN CHANNEL SINUOSITY UPSTREAM OF A LOW-HEAD DAM, HURON RIVER, NORTH-CENTRAL OHIO

The Coho Dam was a low-head (1.5 m tall) dam built in 1969 for fisheries restoration, and removed in 2002. The dam created a small (about 100 m length) reservoir and modified stream gradients >4 km upstream of the dam. These hydrologic modifications created a sedimentation zone upstream of the dam. This study tracks changes in the size, shape, and position of 12 sand-gravel bars within the sedimentation zone from 0.2 km to 4.1 km upstream of the dam, using historical aerial photographs imported into a GIS database.

The individual bars were shown to be highly responsive to hydrologic changes, tending to migrate obliquely downstream and toward the outer bank at rates up to 9 m/yr. Individual bars changed size at rates up to 44% per year (these values were calculated after calibrating for the effect of changing stage height on the exposed surface area of the bars). Collectively, the bars showed a pattern of growth and decay that could be interpreted as the downstream passage of one or more sediment waves.

Channel sinuosity is a significant geomorphologic property that can be pivotal in restoration and reconstruction of fluvial systems. In this case, the primary effects of the dam were to modify channel gradients upstream, resulting in the growth of bars and the increase in the thalweg-path distance through the study area. There was a progressive increase in channel sinuosity from about 1.5 to 1.6 (6%) over the 33-year life history of the dam. Following removal of the dam in 2002, preliminary results suggest a decrease in channel sinuosity. This is interpreted as remobilization of some of the sediment stored upstream of the dam and shortening of the thalweg-path distance.