HISTORICAL CHANGES IN THE HURON RIVER (OHIO): A PARTIALLY BEDROCK-CONTROLLED RIVER

Rivers respond to anthropogenic changes (such as increases in sediment yields) by changing channel geometry, channel pattern, rates of lateral migration or avulsion, and by incision or aggradation. Such responses become more complex when rivers are affected by other controlling factors, such as bedrock geology. Bedrock-controlled rivers are constrained in their ability to adjust to anthropogenic changes, for example rates of incision, channel widening, or lateral migration can be negligible in the presence of non-erosive channel beds and banks. This study focuses on the Huron-Vermillion watershed in north-central Ohio, a region affected by agricultural land clearance from about 1850-present. In the middle portion of the watershed, the Huron River descends the Allegheny Plateau and repeatedly intersects the bedrock surface. The result is a wandering, gravel-bedload stream that consists of alternating bedrock-controlled reaches and alluvial reaches. This study uses historical aerial photographs from 1950-2005 to examine lateral migration rates and processes in the alluvial reaches. Aerial photo sets were scanned at 1200 dpi, georectified using ground control points, and imported into ArcGIS using the 1994 ortho-image as a base. Sequential changes in each of the historical aerial photographs were identified, located, and measured. Ground-truthing was accomplished by vibracoring locations identified on the aerial photos as abandoned channels. Paleohydrologic analysis in the field using micro-topography, vibracore samples, dendrogeomorphology, and recent flood debris suggest that some of the abandon channels are periodically re-occupied during high discharge events. We interpret the complex record of multiple avulsions within the alluvial reaches as due to anthropogenic changes in the sediment budgets, specifically: (1) sediment yield in excess of conveyance capacity caused sediment storage in alluvial channel reaches, (2) there was consequent loss of flood storage capacity in the reach, and (3) this facilitated avulsions during subsequent high discharge events. In contrast, there was little change in the position of the channel through bedrock-controlled reaches. Recognizing the significance of these processes in the alluvial reaches has implications for flood hazard analysis and land management in this and similar rivers.