Effective Sediment-Transporting Discharge from Magnitude-Frequency Analysis

The long-term effectiveness of a geomorphic event of a particular magnitude is the product of the effect of a single event of that magnitude times the frequency with which it recurs (magnitude-frequency analysis). If discharge frequency of a stream is assumed to be log-normally distributed and if sediment transport rate is assumed to be a power function of discharge, then transport effectiveness attains a maximum value at some discharge, termed here the "effective discharge," that moves the most sediment over the long term. The effective discharge and its recurrence interval may be predicted if both assumptions are correct. Analysis of 55 U.S. streams demonstrates that the observed and the predicted recurrence intervals of the effective discharge are in poor agreement. This lack of agreement results from the failure of the power function of discharge to predict adequately the sediment transport rate above some high-discharge threshold or thresholds. The observed recurrence interval of the effective discharge for the studied streams is highly variable, ranging from a week to several decades, calling into question the widely held notion that effective fluvial discharge occurs about once a year.