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FINAL REPORT, November 2003 High-jlow Requirements for the Duchesne River active in the middle part of the study area in 1936 was excluded from the main channel width calculations. Error in calculating subreach channel width stems from error in the determination of channel and bar polygon areas arising from three primary sources: mapping error, digitizing error, and photo distortion. Error in mapping channel area can occur where differences in discharge between various photo series produces the illusion of greater or lesser width of the active channel. This is a potential source of error in this study, as discharge at the time photographs were taken ranged from 8 fe Is to 2,430 ft3 Is (Table 2). However, the mapping scheme employed in this study was designed to minimize this source of error. The 'low bar' mapping unit was applied to the emergent portions of lateral bars and riffles visible at low discharges and other low-elevation bar surfaces. These low bars are progressively submerged at higher discharge so that increasing stage results in both an increase in water area and a corresponding decrease in the area of low bars. As the low-flow channel was defmed to include low bars, these changes have no effect on the area assigned to the low-flow channel as long as flow is contained within the low-flow channel banks. These banks, which separate the low-flow channel area from the higher bar surfaces, are steep and well-defmed, with relief of 0.5 m or more, and can be distinguished on aerial photographs when viewed through a stereoscope. We demonstrate below that bankfull discharge on this portion of the Duchesne River, when the water surface would be expected to cover significant portions of the high bar surface, is greater than 3,000 ft3/s in most locations. Discharges associated with all aerial photographs used in this study are well below this threshold. It was assumed that error in distinguishing high bar surfaces from the low-flow surfaces due to fluctuations in discharge constituted less than 10 percent of the low- flow channel width in most subreaches. A notable exception occurred in subreach 2 on the 1948 photographs where the river had abandoned a long section of its channel and was in the process of incising a new channel farther to the west. Relatively high discharge of the Duchesne River at the time, coupled with backwater effects from the Green River, may have produced a significant exaggeration of channel width through this area of avulsion (Figure 6). The abandoned channel is indicated with a dashed line in the figure. Mapping error associated with the placement of unit boundaries between high bars and other terrestrial units cannot be separated from the subjective nature of these boundaries, which often pass through gradations in vegetation density or elevation lacking sharp breaks. Variability 16