An Investigation of Effective Discharge for Suspended Sediment by Level III Ecoregion

The concept of dominant discharge in alluvial channels was first introduced by Wolman and Miller (1960). In their examination of magnitude and frequency of geomorphic forces, they proposed that the flow transporting the greatest amount of sediment and controlling channel form was not the highest magnitude discharge experienced by a river channel as commonly perceived, but in reality a relatively frequent event. This "effective discharge" can be calculated using flow and sediment-transport data to establish the increment of discharge that transports the largest fraction of the annual sediment load over a period of years. Because of the availability of suspended-sediment data, the dearth of bed-load data and the need to establish water-quality criteria for suspended sediment, this study focuses on the "effective discharge" for suspended sediment only. In the decades since Wolman and Miller's work, the term "dominant discharge" has been associated with two other flows: bankfull discharge, and flow of a given recurrence interval. Pickup and Warner (1976) state the average dominant discharge is the 1.58 year event on the annual flood series, although in other literature the recurrence interval is cited to generally fall between 1.0 and 2.5 years. Conversely, other authors refute that a flow of a universally applicable recurrence interval represents the effective discharge. However, the recent focus on river restoration and rehabilitation projects has meant magnitude-frequency analysis has become more widely applied for designing stable channels, and in many cases the 1.5 year flood has been considered equivalent to the bankfull and effective discharges. The purpose of this paper is to determine the effective discharge for suspended sediment in various ecoregions of the continental United States and to test whether the 1.5 year discharge is a reasonable estimate of this channel-forming flow. To date, the effective discharges for suspended sediment have been calculated for seven ecoregions: Coast Range (1) Sierra Nevada (5), Snake River Basin (12), Arizona-New Mexico Plateau (22), Flint Hills (28), Central Irregular Plains (40) and the Mississippi Valley Loess Plains (74). Mean daily flow data for a substantial period of record were divided into 33 logarithmic classes and the percentage occurrence of each of these was computed. The total annual suspended sediment transport was then calculated for each class, using suspended-sediment rating equations generated through regression of instantaneous discharge and suspended sediment concentration historic sample data. The geometric midpoint of the class transporting the greatest annual sediment load was considered to be the effective discharge. Results to date demonstrate in environments as diverse the Sierra Nevada Mountains and the Mississippi Valley Loess Plains, the effective discharge for suspended sediment is close to the 1.5 year flow. In the former ecoregion, the mean recurrence interval of suspended sediment effective discharge was 1.526 years. The eventual aim of this project is to calculate effective discharges for all 2929 USGS gaging stations with sufficient instantaneous suspended sediment and associated discharges measurements. These sites cover all 84 level III ecoregions, permitting environmental and geographic patterns of effective discharge recurrence interval. Calculations will be repeated using higher temporal resolution (15 minute) flow data.