| Bed load transport is a fundamental physical process in alluvial rivers, building and maintaining a channel geometry that reflects both the quantity and timing of water and the volume and caliber of sediment delivered from the watershed. A variety of formulae have been developed to predict bed load transport in gravel-bed rivers, but testing of the equations in natural channels has been fairly limited. Here, I assess the performance of 4 common bed load transport equations (the Meyer-Peter and Muller [1948], Ackers and White [1973], Bagnold [1980], and Parker [1990] equations) using data from a wide range of gravel-bed rivers in Idaho. Substantial differences were found in equation performance, with the transport data best described by a simple power function of discharge. From this, a new bed load transport equation is proposed in which the coefficient and exponent of the power function are parameterized in terms of channel and watershed characteristics. The accuracy of this new equation was evaluated at 17 independent test sites, with results showing that it performs as well or better than the other equations examined.; However, because transport measurements are typically taken during lower flows it is unclear whether this and other previous assessments of equation performance apply to higher, geomorphically significant flows. To address this issue, the above transport equations were evaluated in terms of their ability to predict the effective discharge, an index flow used in stream restoration projects. It was found that accurate effective discharge predictions are not particularly sensitive to the choice of bed load transport equation. A framework is presented for standardizing the transport equations to explain observed differences in performance and to explore sensitivity of effective discharge predictions.; Finally, a piecewise regression was used to identify transitions between phases of bed load transport that are commonly observed in gravel-bed rivers. Transitions from one phase of motion to another are found to vary by size class, and equal mobility (defined as pi/fi ≈ 1, the proportion of a size class in the bed load relative to that of the subsurface) was not consistently associated with any specific phase of transport. The identification of phase transitions provides a physical basis for defining size-specific reference transport rates (W*ri). In particular, the transition from Phase I to II transport may be an alternative to Parker's [1990] constant value of W*ri =0.0025, and the transition from Phase II to III transport could be used for defining flushing flows or channel maintenance flows. |
