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Average channel gradients of the 15-mile and 18-mile reaches were determined with a <br />mapping grade global positioning system (GPS). Readings of the water, surface were taken at <br />0.8-km intervals along the length of the river. Subsequently, the raw data were corrected with <br />differential post-processing techniques and base-station measurements obtained by the Mesa <br />County Public Works Department. Post-processing of the field data reduces the vertical <br />positional error to t 0.5-0.3 m. These errors tend to be random and are small in comparison to <br />the total drop in elevation through the study reaches (35-45 m). The GPS measurements show <br />that the longitudinal profile of the Colorado River is very smooth between Palisade, CO, and <br />Westwater, UT (Fig. 3). Average channel gradients determined from these measurements are <br />0.00175 in the 15-mile reach; 0.0013 in the 18-mile reach; and 0.0010 in the Ruby-Horsethief <br />Canyon reach. <br />1500 <br />1450 <br />£ 1400 <br />c <br />1350 <br />R <br />m <br />w 1300 <br />1250 <br />1200 <br />300 290 280 270 260 250 240 230 220 210 200 <br />Distance Above Green River Confluence (km) <br />Figure 3. Longitudinal profile of the Colorado River from Palisade, CO, to Westwater, UT. <br />Distances are listed in river kilometers (RKM) above the Green River. Bars below the data <br />indicate the locations of the 15-mile reach (15M), 18-mile reach (18M), and Ruby- <br />Horsethief Canyon reach (RH). The confluence with the Gunnison River is located at RKM <br />275. <br />8