EFFECTS OF A FLOOD CONTROL RESERVOIR ON HYDROLOGY AND CHANNEL MORPHOLOGY OF THE GREEN RIVER OF KENTUCKY

The Green River of Kentucky is an Ohio River tributary influenced by flood control reservoirs, navigation dams, and a long history of forestry and agricultural land use. This study is focused on 20th century hydrological and geomorphic change on the Upper Green River between Green River Lake Dam and the USGS gaging station at Munfordville, KY. The hydrologic and geomorphic effects of the dam at Green River Lake were evaluated with analysis of aerial photos and discharge measurements and streamflow records at Munfordville and other gaging stations. Aerial imagery from several decades over a fifty year period including the construction of the dam were used to estimate changes in channel width and assess changes in riparian land use patterns over a 45 km tailwater reach below the dam. Analysis of discharge measurements and stage-discharge relationships at gaging stations in the tailwater reach provided additional indications of temporal change in channel morphology over the period of record.

Hydrologic analysis of streamflow records revealed significant changes in flow regime due to the flood control reservoir, including a reduction in the frequency and magnitude of peak flows, an increase in frequency of very rapid flow level fluctuations, and an increase in the frequency of very low and moderate in-channel flows. Estimates of channel width changes suggest progressive widening of several meters over the period between 1950 and 1995, with the rate of width increase greater following dam construction. This result is consistent with the likely acceleration of bank erosion by the increased rapidity and frequency of flow fluctuations over a limited in-channel range of stage levels. However, analysis of hydraulic geometry relationships and records of flow measurements at two tailwater gaging stations did not suggest significant changes in channel dimensions or stability at these locations. These results appear to suggest relatively limited adjustments of river morphology in response to hydromodification. Beyond the purely geomorphic response of the river, the combination of increasing width and altered hydrologic regime has likely modified spatial and temporal distributions of hydraulic habitat that may contribute to the decline of native mussel populations in the Green River.