Effective discharge recurrence intervals of Illinois streams

Effective discharge values are routinely used to characterize the geomorphic impact/work that different flows have on streams and watersheds. Effective discharge values are also routinely employed in stream restoration design. Reliable flow frequency and sediment-rating curves are needed to compute effective discharge values, but are not available for many streams. In lieu of regional effective discharge data, researchers/engineers often assume that a stream's effective discharge is the same as the 1.5-year flow event for that stream. However, research shows that this is not always true. Hence, demand exists for developing regional effective discharge values that can be applied at ungaged stream locations. A regional study of effective discharge values within Illinois is performed. Eighty-eight monitoring sites were identified as locations where effective discharge values could potentially be computed. Twenty of these sites were determined to have sufficient data to compute effective discharge values. For these stations, effective discharge values were computed using two different sediment-discharge relationships (a power curve and an alternative “mean approach”). The mean approach estimates effective discharge values that are, on average, 1.9 times larger than those estimated using the power curve. The sediment load carried by flows less than or equal to the effective discharge value are, on average, 2.7 times larger when the mean approach is used. Both the power curve and mean approaches typically yield effective discharge values greater than the stream's mean flow and less than its 1.1-year (annual maximum series) flow event. Some of the computed effective discharge values are less than the stream's mean flow. Computing effective discharge values remains a subjective process with large uncertainties involved. Class interval size, sediment-rating curve fitting techniques, and available sediment data influence effective discharge results. A paucity of monitoring data for Illinois streams draining <518 km2 (where many geomorphic analyses and stream restoration projects take place) hinders the regional applicability of effective discharge results. The uncertainties associated with effective discharge values and the inability to validate effective discharge values as a stream's dominant discharge makes channel design using effective and dominant discharge theory problematic. Suggestions for improving regional effective discharge studies and effective discharge estimates are provided.