Effective Measures of ''Effective Discharge''

The concept of effective discharge, Q_e, introduced by Wolman and Miller in 1960 is that streamflow which transports the most sediment over time. Recently, numerous investigators have questioned how meaningful Q_e is as a descriptor of sediment transport and new discharge indices, such as the half-load discharge Q_1/2, have been introduced to improve the precision with which one describes the ability of a particular discharge to transport sediment. Numerous recent studies have also suggested that among the various empirical estimators of Q_e, there does not seem to be a uniform consensus as to which is most consistent and meaningful. This study uses an example to clarify the method of estimation of both Q_e and Q_1/2. We show that the discharges up to the effective discharge for the Mississippi River at St Louis, are only responsible for carrying 25% of its long term sediment load. We question whether or not such a discharge is truly "effective" in terms of its ability to transport sediment, over time. An alternative index, the half load discharge, is recommended instead, because it is always responsible for transporting 50% of the long term sediment load.