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312 uJ I- ~ ~ I- ~ o a.. Vl Z ~ ~ I- uJO uJUZ I-Z~ ~uJ> ~~U ~~a5 OU:l BigS Zu..~ ~O~ 1->0 ~UI- ZZU uJuJ:l :;:l0 CiSo uJ~~ Vlu..a.. ~a:i0 Effective discharge I /-~ / " I \ I / I I I I I / I y-......., 7 "- ~ ' /1 \ II '\ , I ' I I I / /1 II '/ ,? " '~ " '........, .........., -- -- DISCHARGE -- Fig. 1. Relations between discharge and sediment-transport rate, frequency of occurrence and the product of frequency and transport rate (after Wolman and Miller, 1960). over a period of years is determined \)y two factors - its magnitude and fre- quency. The magnitude or relative significance of a given discharge is repre- sented by its sediment-transport rate, curve A. The sediment-transport rate (lb) commonly varies proportionally with discharge, Q, to a power, n, greater than 1: Ib a: Qn (1) Thus, a doubling of the discharge results in more than a two-fold increase in the sediment-transport rate. The frequency with which a given discharge occurs is represented by curve B, the distribution of streamflows. The most frequent discharges are moderate and transport relatively small amounts of sediment. Conversely, the largest discharges are rare, but transport very large amounts of sediment. The relative effectiveness (curve C) of a given discharge is the product of the sediment- transport rate and the frequency of occurrence. As shown, it is a range of intermediate discharges that transports the largest part of the average annual sediment load. This range of discharge may be represented by the modal sedi- " '.0 ~ . '~ 1 ~. "".. r