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16 Evaluation of Streamflow Lasses Along the Gunnison River from Whitewater Downstream to the Redlands Canal Diversion Dam, near Grand Junction, Colorado, Water Years 199x2003 generally positive differences during the second one-half of the year (fig.lOB). The smaller overall difference (table 2) and the smaller 3-day daily mean differences (fig. 10) for WY 2003, however, partly could be the result of (1) the smaller than nor- mal discharges during WY 2003 (fig. 2); (2) the period of no diversion by the Redlands Canal (fig. 10); and (3) the use of daily mean discharges for the below-Redlands-dam station that were partially quality assured. Streamflow Measurements A reconnaissance of the study reach was made during January 2003 to evaluate the geologic, hydrologic, and physical characteristics of the study reach. The location of geologic fea- tures, such as faults, or man-made features, such as gravel pits, that could affect streamflow loss along the study reach were noted for consideration in analysis of the study results; how- ever, the effects of these on streamflow in the study reach were considered to be very minimal. Most of the study reach is inac- cessible by vehicles, except in the vicinity of the upstream and downstream stations; therefore, the reconnaissance was made by canoes, traveling downstream from the Whitewater station. Four miscellaneous-measuring sites (M1-M4; fig.l, table 1) were selected during the reconnaissance for use in making the discharge measurement sets to aid in evaluating streamflow loss. Discharge measurements at the intermediate locations could help determine if the losses are uniformly distributed throughout the study reach or if the losses are just in a certain location or locations. Two discharge measurement sets were obtained, one dur- ing February 2003, and one during May 2003 (fig. 2). All dis- charge measurements for measurement set 1 were made by wading and using standard current meters (Rantz and others, 1982a). Discharge measurements for measurement set 2 were made by wading (at the below-Redlands-dam station), long rod (at the Redlands-Canal station), and cableway (at the Whitewater station), all using standard current meters (Rantz and others, 1982a), and by using boat-mounted acoustic doppler current profiler equipment (Morlock, 1996; Simpson, 2001) at site M2. Whitewater and below-Redlands-dam stations to verify discharge rating shifts; the Redlands Canal was not in operation, so measurements were not needed at the Redlands-Canal sta- tion. Measured discharges at sites M1-M4 ranged from 527 to 608 ft3/s; measured discharges at the Whitewater station were 628 and 588 ft3/s; and measured discharges at the below- Redlands-dam station were 579 and 565 ft3/s (fig. 11). All mea- surements were rated good (5-percent accuracy), except the measurement of 628 ft3/s at the Whitewater station, which was rated fair (8-percent accuracy). Recorded unit discharges at the Whitewater station ranged from about 575 to 615 ft3/s, and recorded unit discharges at the below-Redlands-dam station ranged from about 560 to 600 ft3/s during the 2-day period (fig. 11). Although the variation in mean or median discharge among the sites seems large (fig. 11), this partly results from the y-axis scale that has a large expansion of a relatively small dis- charge range. Generally, the range of the discharge measure- ments at each site is well within the 5-percent accuracy. For example, assuming a central discharge tendency of 580 ft3/s, at a 5-percent accuracy, a measured discharge could range from 551 to 609 ft3/s. The variation in discharge from one site or sta- tion to another (fig. 11) partly might be the result of differences in measurement technique, but probably is more attributable to local variations in channel conditions. For example, site M1 was just downstream from a pool and riffle sequence and site M4 was just downstream from a large bend where the channel began to broaden. At these sites, some discharge could have been flowing through the unconsolidated gravel adjacent to or under the streambed (underflow), only to be discharged back to the stream some distance downstream, indicated by the rises in discharge at site M2 and at the below-Redlands-dam station. Because of the inherent error in discharge measurements (5 percent for measurements rated good), and because the mean discharge (about 580 ft3/s) at the below-Redlands-dam station was only about 2.5 percent smaller than the mean discharge (about 595 ft3/s) at the Whitewater station it was concluded that there was no measurable streamflow loss along the study reach during measurement set 1. Measurement Set 1 Discharge measurements for measurement set 1 were obtained during February 5-6, near the lowest discharge period during WY 2003 (fig. 2). Discharge was measured 5-8 times over a 24-hour period during the 2-day period at sites M1-M4, by teams who accessed the sites by canoe and remained onsite (from about mid-day on February 5 to about mid-day on February 6). Temporary staff gages also were installed at the four sites, and stage was observed about every hour and more frequently during each measurement. Maximum change in observed stage was small at the sites, ranging from 0.03 to 0.05 foot. Discharge was measured once each day at the Measurement Set 2 Discharge in the Gunnison River during measurement set 2 (May 14-15, fig. 2) was about 2,000 ft3/s and increasing because of high-elevation snowmelt. Because of the high discharge, wading measurements were not possible at any of the sites or stations, except at the below-Redlands-dam station, where discharge was lower due to the upstream diversion into the Redlands Canal. In addition, besides making discharge measurements at the three stations to verify discharge rating shifts, discharge was measured only at site M2 for the following reasons: (1) The changes in discharge observed from one site or station to another during measurement set 1 (fig. 11), likely would not be observed during measurement set 2 because of the