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Analysis of Historical Discharge Records 5 <br />accompanied by minor amounts of residential area and a few <br />gravel pits (U.S. Geological Survey, 1992). <br />Snowmelt during late spring and early summer in the <br />mountainous headwaters of the Gunnison River basin provides <br />most of the streamflow to the study reach. With the exception <br />of occasional monsoonal-thunderstorm events and minor, infre- <br />quent snowmelt, the surrounding highlands adjacent to the <br />study reach contribute little streamflow to the main stem of the <br />Gunnison River (Butler and Leib, 2002). Since major comple- <br />tion ofthe AspinallStorage Unit (1967), the timing and quantity <br />of streamflow have changed at the Whitewater station. Snow- <br />meltpeak flows during April-July are attenuated, and low flows <br />during August-March are larger after completion of the <br />Aspinall Storage Unit in comparison to the more natural flow <br />conditions prior to completion of the unit (fig. 2). Nevertheless, <br />mean annual discharge decreased only slightly from about <br />2,577 ft3/s (1,867,000 acre-ft) during 1916-65 to about <br />2,538 ft3/s (1,838,000 acre-ft) during 1967-2003. Discharge <br />during the study period (WY 2003) generally was less than the <br />10th percentile discharge (for WYs 1967-2003) during the first <br />half of the water year and generally was between the 10th and <br />25th percentiles, except for a few days of higher flow, during <br />the second half of the water year (fig. 2). <br />The geology within and adjacent to the study reach pre- <br />dominantly consists of Mesozoic sedimentary rock formations <br />and Quaternary alluvium (Bankey, 2004). The oldest formation <br />in the area is the Brushy Basin Member of the Jurassic Morrison <br />Formation, which is composed primarily of multicolored clay- <br />stone and mudstone (bentonitic). The Brushy Basin Member <br />outcrops along the western flanks of the area and often is over- <br />lain by Quaternary landslide deposits. The Cretaceous Burro <br />Canyon Formation and the Cretaceous Dakota Sandstone For- <br />mation overlie the Brushy Basin Member throughout the area <br />and form sandstone-topped cliffs along the banks of the river, <br />especially between sites M3 and M4. Jointing in these sand- <br />stones seen during reconnaissance of measuring sites is consis- <br />tent with the northwest trend of faults mapped by Scott and oth- <br />ers (2002). Overlying the Dakota Sandstone, the Cretaceous <br />Mancos Shale Formation outcrops along the riverbanks <br />between the Whitewater station and site M1. Various gravels, <br />alluvium, and colluvium of Quaternary age cover the valley <br />floor, some of which are mined in gravel pits along the river <br />between the Whitewater station and site M1, and between site <br />M3 and the Redlands Canal diversion dam (Scott and others, <br />2002). <br />Acknowledgmerrts <br />The authors extend a special thank you to Jerry Thrush of <br />the CDWR in Montrose; his knowledge of the operation of the <br />below-Redlands-dam and Redlands-Canal stations was very <br />useful in completing the analysis of the historical discharge <br />records, and this analysis could not have been completed with- <br />out his assistance in providing the needed records for the anal- <br />ysis. In addition, his assistance during measurement set 2, <br />especially in measuring discharge at the Redlands-Canal station <br />is greatly appreciated. The assistance of Coll Stanton, BOR in <br />Grand Junction also is appreciated, especially for providing a <br />discharge database for water year 2003 to be included in the <br />analysis of historical discharge. Technical reviews of the report <br />made by George Smith (U.S. Fish and Wildlife Service), and <br />Coll Stanton also are greatly appreciated. <br />Thanks also are extended to these USGS personnel who <br />assisted in completion of the study and preparation of this <br />report: Assistance during measurement sets 1 and 2, especially <br />those who spent a cold night out along the river during February <br />5-6, 2003-Eric Adams, Rich Carver, Dan Chaffin, Paul Diaz, <br />Ben Glass, Dave Hartle, Ken Leib, Paul von Guerard, and Kirby <br />Wynn; additional technical reviews-William H. Asquith and <br />Jack Veenhuis; editorial review-Carol Anderson; preparation <br />of figure 1-Jean Dupree; and preparation of final manu- <br />script-Margo VanAlstine and Alene Brogan. <br />Analysis of Historical Discharge Records <br />As previously discussed, a cursory use of real-time dis- <br />charge data for the three stations along the study reach (fig. 1, <br />table 1) by water-resource managers indicates that the study <br />reach generally is a losing reach. Therefore, a component of the <br />study was a detailed mass-balance analysis of the historical dis- <br />charge records available for the three stations along the study <br />reach. <br />Overview ofDischarge-Records Computation <br />Two types of discharge data usually are available for a <br />gaging station. First, unit discharge data, which typically are <br />data recorded at 15-minute intervals, and second, daily mean <br />discharge data, which are computed from 24 hours (midnight to <br />midnight) of unit-discharge data. Astreamflow-gaging station <br />is equipped to directly measure water-surface (stream) eleva- <br />tion (stage) relative to an arbitrary datum, and a recording <br />device records the stage every 15 minutes. Discharge is indi- <br />rectly measured using stage and astage-discharge relation. <br />When a station is first established, a number of discharge <br />measurements are made at the station at various stages, which <br />are used to develop the stage-discharge relation (discharge rat- <br />ing) that relates a given stage to a specific discharge. The dis- <br />charge rating then is used to determine aunit-discharge value <br />for each of the unit-stage values that are recorded; the daily <br />mean discharge is computed by applying the daily mean stage <br />to the discharge rating or by computing the mean of the unit <br />discharges. After a station has been established and the stage- <br />discharge relation defined, periodic discharge measurements <br />are needed because changes in the channel conditions, such as <br />bed scour or fill during changes in the flow regime, commonly <br />modify the original stage-discharge relation; this is especially <br />relevant to natural stream channels. These additional measure- <br />ments might define corrections, called shifts, that are applied to <br />