RESPONSE OF THE OHIO RIVER FLUVIAL SYSTEM TO CLIMATE CHANGE: A MODEL DEVELOPED FROM QUATERNARY GEOLOGY AND USGS PEAK VALUES

The Ohio River basin's late Quaternary history was dominated by variations in sediment supply, which were controlled primarily by the extent and nature of glaciation and secondarily by colluvial and alluvial processes within the basin. Although most of the basin was unglaciated, voluminous outwash triggered aggradation of the Ohio River and raised base levels for adjacent tributaries. Hence, glacial and base-level events are recorded clearly along the Ohio River, lower reaches of its major tributaries, and glaciated streams in the basin. In contrast, non-glacial impacts of climate on the system were subtle and best understood through modern analogs.

USGS Peak Value data show that the ~2000 gauged streams in the Ohio River basin respond to atmospheric events in complex, but identifiable patterns. The Ohio main stem and large northern tributaries experience record flows from rain-on-snow-melt events, but large southern tributaries respond to frontal precipitation. Small streams throughout the basin are likely to set discharge records during convectional storms in late spring and early summer. Late summer and fall are relatively flood-free, except for tropical cyclones that impact <10,000 sq. km watersheds in southeastern parts of the basin.

Over 28 percent of peak flows have been recorded during six weeks between 1 March and 10 April, when moist maritime tropical air from the Gulf of Mexico clashes with cold arctic air over the basin. In southern streams, these floods stem from severe weather and high-intensity precipitation, whereas snow-melt usually contributes to northern flooding in this season. Floods in middle latitudes of the basin are more uniformly distributed throughout the year.

The peak-flow data suggest that small streams responded to climate out of phase with large streams. Sediment transport on large rivers may have reached maximum efficiency during late glacial snow-melt and frontal precipitation events, while small streams may have been most efficient during middle Holocene convectional storms or, in the Southeast, tropical cyclones.