Examination, application, and evaluation of geomorphic principles and resulting water quality in Midwest agricultural streams and rivers

This dissertation contains research designed to evaluate geomorphic principles, predominately channel-forming discharges, and their application and performance in agriculturally dominated headwater streams. Measured data were used to evaluate whether bankfull discharges were related to effective discharges for large rivers in Ohio. The frequency and sediment transport associated with these channel-forming discharges were also examined. Bankfull discharges were determined by measuring fluvial features at each USGS gage and then relating these features to the rating curve and historic daily discharge data for each gage. Effective discharges were determined by using suspended sediment data obtained at the gages, the Wolman-Miller method for calculating geomorphic work, and bin sizes based on stage intervals to group sediment and discharge data. There was good agreement between the effective discharge and bankfull discharge estimates. Recurrence intervals of the bankfull and effective discharges ranged from 0.3 to 1.4 years. These recurrence intervals are more frequent than generally reported in the literature. The duration of daily discharges that equaled or exceeded the channel-forming discharge ranged from 1 to 24 days annually, with mean values of 9 and 11 days for the bankfull discharge and effective discharge, respectively.; An approach was developed to size and modify agricultural drainage ditches in the Midwest to a two-stage geometry to maintain drainage function and capacity while increasing channel stability. Two-stage channels consist of a main channel and small floodplain (benches) within the ditch confines. Floodplains are geomorphic features that dissipate energy, reduce the erosive potential of high flow volumes, increase ecological habitat, and improve water quality. The two-stage channel sizing and assessment procedure includes nine steps: (1) project identification; (2) data collection; (3) data analysis; (4) conceptual channel system sizing; (5) hydrologic evaluation; (6) project assessment; (7) design and/or final sizing; (8) construction; and (9) monitoring and assessment of performance. Channel width and depth dimensions are determined by taking detailed measurements at the project site, stable reference site, and throughout the watershed; these measurements are verified by comparing them with calculated channel relationships. The authors have developed spreadsheet tools to evaluate the geomorphology of two-stage channels. Constructing a two-stage channel requires more excavation than traditional ditch maintenance; however, anticipated benefits include improved conveyance capacity, channel stability, ecological habitat, and water quality. (Abstract shortened by UMI.)...