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326 ability of data on sediment loads, lack of consistent and clearly defined chan- nel features, or uncertainty whether the stream are in quasi-equilibrium. In order to evaluate the geomorphic significance of the effective discharge, 3-5 cross-sectionS were surveyed in the vicinity of each gaging station. The cross-sections were located in a self-formed reach of channels with a well- developed floodplain, so that the bankfull discharge could be identified. The elevation of the floodplain was taken to be the level of the bankfull stage. Secondary geomorphic and botanic features were also noted to verify that the surface was, in fact, the floodplain rather than a terrace. These features include the top of point bars and the lower limit of vegetation. The flood- plain elevations were plotted to give a longitudinal profile of the bankfull stage through the reach of channel including the gaging station. The bankfull discharge was then determined from the stage-discharge relation of the gag- ing station. A typical cross-section of the Little Snake River near Dixon, Wyo., is shown in Fig. 10. The effective discharge and the bankfull discharge are nearly iden- tical. The mean-annual discharge is about 12% of the bankfull and effective discharges. The close agreement between the effective and bankfull discharges indicates that the stream channel is adjusted to the range of discharges that transport the largest part of the annual sediment load. Baker (1977) suggested that relatively rare floods may be the significant 34.5 (I) a: w ~ 34.0 ~ z Bankfull discharge =114 cubic meters per second Effective discharge = 113 cubic meters per second ~ 33.5 I- <( o ~ 33.0 <( a: I- iij ~ 32.5 w > o 1XI <( 32.0 z Q I- ~ 31.5 w -l W 31.00 5 10 15 20 25 30 35 DISTANCE, IN METERS 40 45 50 Fig. 10. Cross-sectional profile of the Little Snake River near Dixon, Wyoming, showing the elevation of the mean-annual, effective and bankfull discharges. 55 ~ " -. . .