EVALUATING CHANNEL-FORMING DISCHARGES: A STUDY OF LARGE RIVERS IN OHIO

Abstract

Measured data were used to evaluate whether bankfull discharges were related to effective discharges for largerivers in Ohio. The frequency and sediment transport associated with these channel-forming discharges was also examined.Rural watersheds in the Midwest region of the U.S. are dominated by agricultural land uses that incorporate subsurfacedrainage improvements. Bankfull discharges were determined by measuring fluvial features at each USGS gage and thenrelating these features to the rating curve and historic daily discharge data for each gage. Effective discharges weredetermined by using suspended sediment data obtained at the gages, the Wolman-Miller method for calculating geomorphicwork, and bin sizes based on stage intervals to group sediment and discharge data. There was good agreement between theeffective discharge and bankfull discharge estimates. Bankfull and effective discharges were primarily related to flows thattransported the middle 50% of the total sediment load. Recurrence intervals of the bankfull and effective discharges rangedfrom 0.3 to 1.4 years. These recurrence intervals are more frequent than generally reported in the literature. The durationof daily discharges that equaled or exceeded the channel-forming discharge ranged from 1 to 24 days annually, with meanvalues of 9 and 11 days for the bankfull discharge and effective discharge, respectively. Common methods for determiningthe recurrence interval are inadequate for frequent channel-forming discharges, and better insight is obtained by determiningthe number of days on which these flows are exceeded annually.