RESPONSE OF UPSTREAM SAND & GRAVEL BARS TO THE CONSTRUCTION AND SUBSEQUENT REMOVAL OF A LOW-HEAD DAM, COHO DAM, HURON RIVER, OHIO

The Huron River is a "wandering gravel channel" through the study area. Eight sets of historical aerial photographs (1958-2003) span the construction of Coho Dam (1969), opening of its weir spillway (1994) and removal of the dam (2002). Twelve sand-gravel bars were identified between 0.2-4.1 km upstream of the spillway. The aerial photographs were co-registered into DOQ's and imported into ArcGIS. Polygons were drawn around each bedform and its centroid calculated. In order to compare photographs from different years, the size (area) of the bedforms on each photograph was adjusted to a standard reference stage height.

The pre-dam condition of the Huron River through the study reach consisted of alternating sedimentation and erosion zones. Individual bedforms showed a preferential oblique migration downstream and toward the outer bank. Spatial and temporal changes in the size of the bedforms indicate that a sediment wave moving downstream through the study reach became stationary as a result of channel adjustments following construction of the dam. The sediment wave subsequently attenuated in situ by dispersion in response to erosion of the upstream reaches and deposition into the new reservoir. Subsequent removal of the weir boards and removal of the dam rejuvenated movement of a sediment wave through the study reach.

The centroid position for each bedform at any given time interval was measured as the thalweg-path distance to a fixed point. After dam closure, the centroids for bedforms >0.8 km upstream of the dam shifted upstream (channel deposition and bar growth increased the sinuosity of the thalweg-path length). After dam removal, bedform centroids shifted back downstream (channel scour and cutting of chutes decreased the sinuosity of the thalweg-path length). The results emphasize the dynamic quality of bedforms upstream of even a fairly small dam.