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References Cited 21 between the moving 3-day daily mean discharges at the down- stream and upstream stations during WYs 1995-2002 were between about -200 and +100 ft3/s during about one-half of the time [within the inter-quartile range (IQR)], but had absolute values as large as about 500 to 1,000 ft3/s during the other one- half of the time (outside the IQR). Percentage differences almost always were between 0 and -10 percent within the IQR and were as small or large as about -60 to +50 percent outside the IQR. Recorded discharge data for WY 2003 also were analyzed after the end of the water year because use of the real-time discharge data for the year seemed to indicate a change in the previously observed trend of losing streamflow along the study reach. The annual difference between downstream and upstream discharge and the percentage difference for WY 2003 were substantially smaller than for any of the previously ana- lyzed years (WYs 1995-2002). WY 2003 was the only year for which the medians were near zero. In addition to a smaller annual difference for WY 2003, the variability in the differ- ences between downstream and upstream discharge also was much less during WY 2003 than during the previous years. Two discharge measurement sets were obtained, one during February 2003, and one during May 2003. Discharge measurements for measurement set 1 were obtained during February 5-6, near the lowest discharge period during WY 2003. Discharge was measured 5-8 times over a 24-hour period during the 2-day period at sites M 1-M4. 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 ft. Discharge was measured once each day at the 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 station. 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. 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. Because of the expected 5-percent differences in dis- charge measurements, and because the mean discharge at the below-Redlands-dam station, about 580 ft3/s, was only about 2.5 percent smaller than the mean discharge at the Whitewater station, about 595 ft3/s, it was concluded that there was no mea- surable streamflow loss along the study reach during measure- ment set 1. Discharge in the Gunnison River during measurement set 2 (May 14-15) was about 2,000 ft3/s and increasing because of high-elevation snowmelt. Discharge measurements were made at the three gaging stations to verify discharge rating shifts. Five discharge measurements also were made at site M2 and ranged from 1,668 to 2,117 ft3/s. Discharges measured at the gaging stations were 2,730 ft3/s (on May 16) at the Whitewater station, 1,268 ft3/s at the below-Redlands-dam sta- tion, and 819 ft3/s at the Redlands-Canal station. Because of the change in discharge during measurement set 2, consideration of traveltime was of critical importance in evaluating any streamflow losses. In a hydrographic analysis of unit discharges during May 14-15, and using an estimated trav- eltime of about 1.5 hours, the discharge measurements made at site M2 correlated closely with the discharges recorded about 1.5 hours earlier at the Whitewater station. Using an estimated traveltime of about 3.5 hours, the sum of the unit discharges at the below-Redlands-dam and Redlands-Canal stations also correlated closely to the unit discharges recorded about 3.5 hours earlier at the Whitewater station. These results for measurement set 2 also indicate no measurable streamflow loss along the study reach. On the basis of the study results, it cannot be concluded that the study reach is, in fact, a losing stream reach because 1. The lack of a final, quality-assured discharge record for the below-Redlands-dam station does not provide a quantita- tive estimate of the error in that discharge record. 2. Even without any quantitative estimate of the error in the discharge record for the below-Redlands-dam station, the. annual water-year differences between downstream and upstream discharge mostly are less than 5 percent and within the stated 5-percent accuracy of discharge measurements and the finalized discharge records for the Whitewater and Redlands-Canal stations. Although some differences between the sum of the daily mean discharge at the downstream stations and the daily mean discharge at the upstream station are large, some of the differences are attributable to traveltime of streamflow through the reach, which is not entirely accounted for in the discharge record, especially in the daily mean values. References Cited Bankey, Viki, 2004, ed., Resource Potential and Geology of the Grand Mesa, Uncompahgre, and Gunnison (GMUG) National Forest and Vicinity, Colorado-Summary informa- tion of known mineral resources: U.S. Geological Survey Bulletin 2213, 276 p. [Available on-line at URL http: //pubs.usgs. gov/bul/b2213 ] Butler, D.L., and Leib, K.J., 2002, Characterization of Selenium in the Lower Gunnison River Basin, Colorado, 1988-2000: U.S. Geological Survey Water-Resources Investigations Report 02-4151, 26 p. Helsel, D.R., and Hirsch, R.M., 2002, Statistical Methods in Water Resources-Hydrologic Analysis and Interpretation: U.S. Geological Survey Techniques of Water-Resources Investigations, book 4, chap. A3, 510 p.