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.' !i y~ f1rlO(C~\i\IS J L. tj. j (I ~' C /YII"'fA/- /~):;;"""/ O:-JJ,C/1 Myj,,,~y ._.,... IL....)v Journal of Hydrology, 46 (1980) 311-330 311 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands . [1 ] EFFECTIVE AND BANKFULL DISCHARGES OF STREAMS IN THE YAMPA RIVER BASIN, COLORADO AND WYOMING EDMUND D. ANDREWS U.S. Geological SurlJey, Lakewood, CO 80225 (U.S.A.) (Received July 16, 1979; revised and accepted September 11, 1979 ABSTRACT Andrews, E.D., 1980. Effective and bankfull discharges in the Yampa River basin. Colora- do and Wyoming. J. Hydro!.. 46: 311-330. r The effective discharge is defined as the increment of discharge that transports the \ largest fraction of the annual sediment load over a period of years. Increments of the aver- \.. age annual total sediment load transported by various discharges were calculated by the flow-duration, sediment-transport-curve method for 15 gaging stations in the Yampa River basin of Colorado and Wyoming. A total sediment-transport curve was constructed for each gaging station by adding measured instantaneous suspended-sediment discharges to bedload-sediment discharges computed by the Meyer-Peter and Mueller relation. The stream- flow durations were compiled from the respective gaging-station records. The quantity of sediment transported by discharges having various frequencies may be computed by com- bining these two relations. The 15 gaging stations had diverse hydraulic and sediment characteristics. Contributing drainal[e area ranged from 51.8 to 9,960 km2, and mean-annual discharge ranged from 0.040 to 43.9 m 3/s. The median diameter of bed material ranged from 0.4 to 86 mm. Mean-annual sediment load from the drainage basins studied ranged from 500 to 1.3.10' metric tons per year. ~ The effective discharges at the U; ,agin, statien,s were equaled or exceeded on the aver- age of between 1.5 days per year (0.4% of the time) and 11 days per year (3.0% of the time). The recurrence interval of the effective discharges ranged from 1.18 to 3.26 yr. on l1e annuaUlood series. To compare the effective discharge with the bankfull discharge, cross-sections were surveyed in a self-formed reach of the channel in the vicinity of each gaging station. The bankfull discharge was defined as the discharge which filled the chan- II, \11, nel to the level of the floodplain. At all gaging stations, the effective discharge and the I ,.{:, ~ bankfull discharge were nearly equal. Thus, the stream channels appear to be adjusted to ~eir effective discharge. INTRODUCTION The concepts of magnitude and frequency of geomorphic forces were de- veloped by Wolman and Miller (1960). Two significant principles of quantita- tive geomorphology were presented. The first principle is that the effective geomorphic force is a relatively frequent event as shown conceptually in Fig. 1 using the example of river mechanics. The effectiveness of a given discharge