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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 1-8 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. There were 4 significant avulsions recorded during the interval 1950-2015, occurring between 1950-1960, 1960-1969, and 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. Data Sources and Properties Fifteen historical images of the Huron River area and 18 historical topographic maps were collected from the Bowling Green Library and the USGS (Figure 1). The historical images were obtained through the USGS, USDA, HistoricalAerials.com, and the Ohio Geographically Referenced Information Program. USGS photographs had scales and elevations as well as the date the photograph was taken. The historical aerial photographs from HistoricalAerials.com had dates flown, elevations of the photograph and the scales. Some of the images were from unknown sources but contained the date of the photograph. GIS A 2015 digital orthoquadrangle map (DOQ) of the Huron River was acquired from the Ohio Geographically Referenced Information Program (OGRIP). The image was pre-projected in Universal Transverse Mercator (UTM ) coordinates (R17 NAD 1983). The resolution was 1 meter by 1 meter cell sizes. This image was used as a base image for georeferencing the historical aerial photographs. Historical aerial images were entered into ArcGIS and projected into UTM coordinates. These images were then georeferenced to the 2015 DOQ image using ground control points.. The RMS error was calculated by comparing the change in position of the X and Y coordinates in the historical images to their final position when georeferenced. The RMS errors ranged from 0.05 to 8.09 meters with and average RMS error of 4.38 meters. Shapefiles were created using the polygon tool in ArcGIS for each of the historical images. These shapefiles were overlain on top of each other as seen in figure 4, and if the size of the avulsion was greater than the RMS error calculated it was considered a true avulsion. Four avulsions were found to have occurred from 1950-1979 within the study area (figure 4). Sinuosities were calculated in ArcGIS using the measure tool to measure the path length and feature length of the stream channel for each of the shapefiles. The average sinuosity between 1950 and 2015 for this section of the Huron River was found to range from 1.64 to 1.79 with an average sinuosity of 1.78 (table 1). The sinuosity decreases from years where avulsions were observed. This is likely due to the channel adjusting to a straighter path following the avulsion (table 1). Distance of the avulsions from bedrock reaches was also calculated using the measure tool in ArcGIS. The distances downstream ranged from 190-220 meters away from the bedrock. The Huron River shows dynamic change in channel position from the 1950s to 2015. The georeferenced historical aerial images allowed for the creation of shapefiles which were used to show the change in channel position from one set of images to another. These shapefiles were overlain on a base image and the RMS errors are able to be used to calculate the size of error between photographs. Using my preliminary results the RMS errors for the historical aerial photographs from 1950 to 2015 ranged from .05 meters to 8 meters. Further work can be done to minimize these errors by changing tie points between photographs. The range of error is small enough to see major changes in the channel allowing for mapping of avulsions.

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