Effective Measures of “Effective” Discharge

The concept of effective discharge, Qe, was introduced by Wolman and Miller (1960) as that stream discharge that transports the most sediment over time. Recently, the validity of Qe as an overall descriptor of sediment transport has been questioned because of its various interpretations and methods of calculation. A new discharge index—termed the “half-load discharge,” Q1/2—was introduced as the discharge above and below which half of the total sediment load has been transported over time. Existing methods along with long records of continuous daily suspended sediment and discharge data are used to clarify what a discharge index can accomplish. We also offer an objective and consistent empirical methodology for calculating Qe based on a kernel density function. Generalized graphical relationships are introduced for summarizing the frequency and magnitude of both stream discharge and sediment loads. We document how Qe and Q1/2 are related to standard descriptors of river discharge as well as general characteristics of the sediment load-discharge relationship.