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FINAL REPORT, November 2003 High-jlow Requirements for the Duchesne River 1999). A USGS gaging station immediately downstream from the Duchesne River -Uinta River confluence, Duchesne River near Randlett (USGS station number 09302000), provides a continuous record of stream flow dating to 1943. Mean annual flow at the gage is 558 fels and the mean annual peak flow is approximately 4,310 fe Is for the period 1943 to 2000. The maximum recorded instantaneous peak discharge was 11,500 fe/s, and occurred on June 20, 1983. Although no significant tributaries enter the Duchesne River downstream from the gage, irrigation diversions and return flow ditches are present in this segment of the river. The lower Duchesne River valley is approximately 2 km wide and is bounded on either side by gravel-capped bluffs in excess of 100 m in height. The relatively flat valley floor contains three distinct terrace surfaces, some of which functioned as active floodplains within historical time, as indicated by extant vegetation, early aerial photography, and early cadastral surveys. Through much of the study area, the valley bottom is best described as a wandering gravel-bed river floodplain (order B2), using the classification system of Nan son and Croke (1992). The active floodplain, which now occupies less than a quarter of the full valley width, is composed primarily of sandy scroll bar platforms with areas of dense vegetation farther from the main channel. Chute channels commonly define the boundary between the bars and vegetated floodplain surfaces. Numerous abandoned channels and swales are present on the floodplain, as well as on the lower terrace levels. Riparian vegetation consists primarily of dense tamarisk thickets (Tamarix spp.), russian-olive (Elaeagnus angustifolia), and young cottonwood (Populus spp.). The lower terrace levels contain decadent cottonwood galleries mixed with sagebrush (Artemesia spp.), rabbitbrush (Chrysothamnus nauseosus), squaw bush (Rhus trilobata), and greasewood (Sarcobatus vermiculatus). The river has a meandering planform, a mixed bed of cobbles, gravel, and sand, with an average gradient of about 0.0019. This general morphology extends through the upstream portion of the study area to approximately river km 9, where the channel slope abruptly decreases to less than 0.0003, bed material changes to sand, and channel form assumes a narrow canal-like geometry oflow sinuosity. The floodplain in this downstream portion of the study area is a comparatively featureless plain dominated by expansive tamarisk thickets. It is best described as either a lateral migrationlbackswamp floodplain (order B3c) or a laterally stable, single-channel floodplain (order C I). Three reaches within the study segment were selected for detailed geomorphic study and hydraulic modeling. These are the 24-hour Camp reach, the Above Pipeline reach, and the 6