Effective discharge and gravel-bed rivers

Effectiveness in geomorphology may be associated with transport (work) and with landform. Bedload transport may relate to both. Determination of that discharge which transports more bedload than any other entails assessment of the magnitude and frequency of bedload transport. This involves arraying daily mean discharges, computing the bedload for each unit of discharge, and determining the discharge that transports the most bedload. For those rivers in which the relationships of bedload discharge to water discharge are not dominated by very large-size particles and where daily mean discharges are an adequate description of streamfow characteristics, a large body of data demonstrates that this discharge is often analogous to bankfull discharge and often has a frequency of recurrence of one to two years. Rivers with constraints on the mobility of bed material, typically armoured channels with steeper than average relationships of bedload discharge to water discharge, may have an effective discharge occurring at higher stages and greater recurrence intervals than bankfull discharge. Bedload discharges were measured at live snow-melt dominated, gravel-bedded rivers in the northern Rocky Mountains, USA: drainage areas at the sites ranged from about 55 to 4950 km(2) and bankfull discharges ranged from about 6.5 to 650 m(3) s(-1). Median bed-surface particles, d(50), ranged from 40 to 173 mm, and d(90) ranged from 132 to 310 mm. Exponents of the bedload ratings, indicating the steepness of the relationship of bedload discharge to water discharge, ranged from 2.30 to 5.06. Values of the exponent correlated well with values of either d(50) or d(90) of the bed-surface material and demonstrate the effect of the bed-surface material on the bedload rating. Effective discharge was determined for each site. The ratio of effective discharge to bankfull discharge. Q(e)/Q(b), ranged from 0.98 to 1.31. The values of Q(e)/Q(b) correlated well with exponent values from the bedload ratings. Data indicate that, as the exponent of the bedload rating increases from typical to more steep values, effective discharge increases from near-bankfull discharge to, in the present study, about 1.3 bankfull discharge. This represents about a doubling of the recurrence interval. Copyright (C) 2001 John Wiley & Sons, Ltd.