Abstract. The relationship between flow and bed load transport measured for 10 years in six gravel-bed streams in Idaho exhibits annual hysteresis. At a given flow rate, more bed load is carried by discharges preceding the first annual occurrence of a "threshold" rate, which is characteristic of each stream. Incorporating the effect of hysteresis leads to a small improvement in the fit of the bed load-flow regression. As the turning point for hysteresis, a constant threshold discharge is found to work better than the annual peak discharge. This bimodal hysteresis model is also found to out perform one with a more gradual transition, based on cumulative discharge. These results are interpreted to reflect a buildup of readily moved sediment supplies during the low-flow periods from late summer to early spring, supplies which are then exhausted by rising springtime discharges up to the threshold. The threshold is greater than mean annual discharg e and about onehalf bank-full discharge.

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