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
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