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FINAL REPORT, November 2003 High-jlow Requirements for the DuchesneRiver roughness parameters were adequately specified, any error in that initial water surface elevation tended to diminish for predicted energy heads at more upstream cross sections. Longitudinal variability in discharge caused by irrigation withdrawals and returns through the lower Duchesne River is a potential source of error in evaluating stage-discharge relationships at the modeling sites. The Wissiup Ditch diverts a potentially significant portion of flow from the river just downstream from the 24-hour Camp site. Part ofthis water is pumped out of a floodplain pond, part returns to the river at about river Ian 16.5, and a large portion re- enters the river at the downstream end of the Wissiup Return site. Another large ditch, the Leland Canal, diverts an unknown quantity of water upstream from the gage near Randlett. Some portion of this water re-enters the river as diffuse return flow downstream from the gage. Deviation in discharge at the modeling sites as compared to the discharge recorded at the gage near Randlett was evaluated by measuring discharge at each of the study sites. In addition, discharges calculated from 21 channel transects with slope and velocity measurements supplied by US Fish and Wildlife personnel were compared to the discharges recorded at the gage. Although interpretation of these data was complicated by problems with unsteady discharge at the gage and distance from the gage, the data suggest that longitudinal discharge variation through the study area is generally within about 70 ft3/s of the gaged value during the irrigation season. This magnitude of variability represents a significant proportion of base flow, but is relatively small compared to peak flow. Cross section data for building the Green River backwater model was mainly extracted from a 2-m grid of river bed elevations developed by JC Headwaters, Inc. This grid was produced using DGPS-integrated hydro-acoustic technology in June 1999. Fifty-nine cross sections extending from 0 to 9600 meters upstream from the confluence with the Green River were extracted from the grid using an Avenue script in Arc View@ GIS. Two of the extracted cross sections were selected so as to coincide with a pair of cross sections we surveyed on the lower Duchesne River during the 2000 field season. The locations of these cross sections (site 4 and site 5) are indicated on Figure 2. Cross sections extracted from the grid portray the submerged bed of the river only - banks are omitted. Comparison of the extracted cross sections with surveyed cross sections allowed reconstruction of the bank heights above the channel bed. As the entire downstream sand-bedded portion of the river exhibits simple canal-like channel 26