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Introduction 3 discharge data are in operation (as of 2003) in the reach. During 1995-2002, use of real-time discharge data for these three sta- tions seemed to indicate that the sum of the discharges at the two downstream stations (a main channel and a canal station) usually was less than the discharge at the upstream sta- tion (only amain-channel station), indicating a general loss of streamflow in the reach (Coll Stanton, Bureau of Reclamation, and Jerry Thrush, Colorado Division of Water Resources, oral commun., 2002). The Gunnison River also is important in the Upper Colorado River Endangered Fish Recovery Program (Recovery Program) (U.S. Fish and Wildlife Service, 2002a). The Recovery Program, established in 1988, was implemented to conduct along-term recovery program for four endangered upper Colorado River basin fish species-the Colorado pike- minnow, razorback sucker, bonytail, and humpback chub-that are protected under the Federal Endangered Species Act of 1973. These endangered fish species once thrived in the Colorado River system, but water-resource development, including the building of numerous diversion dams and several large reservoirs, and the introduction of nonnative fish resulted in large reductions in the numbers and range of the four species (U.S. Fish and Wildlife Service, 2002b). In March 1994, the Department of the Interior designated 1,980 miles of the Colorado River as "critical habitat" for the Colorado pikeminnow, razorback sucker, bonytail, and hump- back chub (U.S. Fish and Wildlife Service, 2002c). In the vicin- ity ofGrand Junction, the designated critical habitat includes (1) the Gunnison River from Delta downstream to the confluence with the Colorado River at Grand Junction, (2) the Colorado River from the Gunnison River confluence upstream to the Grand Valley Canal diversion dam (the "15-mile reach"), and (3) the Colorado River from the Gunnison River confluence downstream to about Salt Creek (the "18-mile reach"). [Note: Delta, Grand Valley Canal, and Salt Creek are outside of the study area and are not shown in figure 1]. Movement of endangered fish from the 15- and 18-mile reaches on the Colorado River into the Gunnison River reach has not been possible since about 1917, when the existing Redlands Canal diversion dam (fig. 1) was completed. In 1996, a 350-foot fish ladder was built by the Recovery Program at the Redlands Canal diversion dam; the ladder provides upstream access to 57 miles of historical habitat that had been inaccessi- blefor nearly a century (U.S. Fish and Wildlife Service, 2002d). However, during low-flow conditions [about 1,000 cubic feet per second (ft3/s), or less] in the Gunnison River, diversion by the Redlands Canal, which has a decreed capacity of 850 ft3/s, might decrease discharge flowing through the fish ladder to the point that fish cannot move upstream. The ladder requires 100 ft3/s to operate and an additiona1200 ft3/s are needed to help the fish navigate through shallow areas downstream from the dam (U.S. Fish and Wildlife Service, 2004). The Recovery Program has identified recommended flows for the Gunnison River. The BOR intends to prepare a draft Environmental Impact Statement to describe potential effects of operational changes to the Aspinall Unit and its authorized purposes that are related to compliance with the Endangered Species Act (Coll Stanton, Bureau of Reclamation, written commun., May 3, 2004). An important aspect of any opera- tional change is the resulting change to the Gunnison River; therefore, it is necessary to understand the streamflow-loss characteristics of the Gunnison River, particularly along the reach from Whitewater downstream to the Redlands Canal diversion dam, which is thought to be a losing reach. To better understand the streamflow-loss characteristics of this reach, the U.S. Geological Survey (USGS) began a study in 2003, in cooperation with the Colorado Water Conservation Board, the Recovery Program, the Colorado River Water Conservation District, the CDWR, and the BOR, to characterize streamflow losses in the reach of the Gunnison River from Whitewater downstream to the Redlands Canal diversion dam. Results of the study could help in refining operational release rates from the Aspinall Storage Unit to support the Recovery Program, as well as release rates for other purposes. Purpose and Scope The purpose of this report is to describe the methods and results of a study of the streamflow-loss characteristics of the Gunnison River from Whitewater downstream to the Redlands Canal diversion dam. Specifically, this report describes the methods and results of the two principal components of the study that include (1) a detailed mass-balance analysis of historical discharge records that were available for the three gaging stations along the study reach, and (2) two sets of dis- charge measurements that were made at the three stations and at four additional sites. For the mass-balance analysis of historical discharge records, data for these current (2003) gaging stations (fig. 1, table 1) were compiled and analyzed: (1) Station 09152500 Gunnison River near Grand Junction (hereinafter the "White- water" station), operated by the USGS; (2) Station GUN- REDCO Gunnison River below Redlands Canal diversion dam (hereinafter the "below-Redlands-dam" station), operated by the CDWR; and (3) station RLCGRJCO Redlands Canal near Grand Junction (hereinafter the "Redlands-Canal" station), also operated by the CDWR. Data for water years (WY) 1995-2003 were used for the mass-balance analysis, because data for the below-Redlands-dam station were available only for this period (table 1). For WY 1995, data for the below-Redlands-dam station only are available for March 1-September 30, but in this report, the period is referred to as "WY 1995." Although the below-Redlands-dam and Redlands-Canal stations are about 0.7 mile downstream from the Redlands Canal diversion dam (fig. 1, table 1), streamflow losses were investigated only for the reach from the Whitewater station downstream to the Redlands Canal diversion dam (fig. 1). It was assumed that the sum of the discharges at the below-Redlands- dam and Redlands-Canal stations, whether recorded or mea- sured, was the same as the discharge in the Gunnison River just upstream from the diversion dam; hence, for purposes of the