AVULSION PROCESSES AND RATES IN A MIXED ALLUVIAL-BEDROCK RIVER, HURON RIVER, NORTH-CENTRAL OHIO (U.S.A.)

Bedrock exerts strong control over fluvial channel migration and channel-switching events (avulsions). This study looked at a river with alternating reaches of bedrock and alluvium to determine the downstream zone of influence that bedrock reaches have on the locations of avulsions in subsequent alluvial reaches. The study area is an 8-km stretch of the Western Branch of the Huron River within the Milan Wildlife Area near Norwalk, Ohio. In the study area, the Huron River has channel bed and banks of the Paleozoic Ohio Shale alternating with Pleistocene glacial till and Holocene colluvium and alluvium. Historical aerial photos from 1950, 1956, 1960, 1969, 1977, 1988, 1995, 2000, 2001, 2004, 2013 and 2015 were collected from the USGS and the Ohio Geographically Referenced Information Program (OGRIP). These were georeferenced using ground control points, input into ArcGIS, and the RMS error was calculated to be between 2-4 meters when comparing each historical image to the 2015 digital orthoquadrangle map (DOQ) acquired from OGRIP and used as a base map. The historical aerial photos were used to find when and where avulsions occurred, as well as changes in channel path length and sinuosity following avulsions. The 4 significant avulsions that occurred between 1950-2015 were between 1950-1960, between 1960-1969 and two occurred between 1977-1979. Typically, the exit point from the previous channel was located 190-220 meters downstream of the bedrock reach immediately upstream. One mode of bedrock control is that avulsion direction (azimuth) appears to be controlled by the underlying bedrock joint sets (one set is oriented approximately N-S and another set oriented approximately E-W). Over the interval 1950-2015, sinuosity varied from 1.6 to 1.8, generally increasing to a maximum value prior to an avulsion. To better characterize the avulsion events, eleven trenches up to 2 meters deep were dug at 6 locations along the Huron River and 8 vibracores were collected at one avulsion site. On-going work will link avulsions to known hydrological events from the historical record at a downstream gaging station, and will examine the sedimentological record of cut-and-filling in the avulsion channels.