Evaluation of Data from Two Gunnison River Temperature Monitoring Stations Discontinued in 2018 by the Program Director’S Office

Evaluation of Data from Two Gunnison River Temperature Monitoring Stations Discontinued in 2018 by the Program Director’s Office Jana Mohrman and Don Anderson Upper Colorado River Recovery Program January 2020 Purpose of Report The Upper Colorado River Endangered Fish Recovery Program (Recovery Program) has been monitoring river water temperatures at multiple sites in the Gunnison River basin since as early as 1992 (Figure 1 and Table 1) for multiple purposes, including evaluating the influence of the Bureau of Reclamation’s Aspinall Unit on downstream river temperatures and the commensurate effect on fish habitat conditions. This report documents the rationale for discontinuing, in 2018, two sites originally established in 1996, and provides some analysis of the information those sites provided. Gunnison River Temperature Monitoring The Bureau of Reclamation’s Aspinall Unit consists of a series of reservoirs on the mainstem Gunnison River including Blue Mesa Reservoir (established in 1966; storage capacity 940,800 acre- feet) and two smaller reregulating reservoirs downstream, Morrow Point (1968) and Crystal (1977). Temperatures in the lower Gunnison River are influenced to some degree by the temperature of water passing through the Aspinall Unit. The Recovery Program has annually monitored 12 sites in the Gunnison River basin below the Aspinall Unit; those sites have been monitored between 13 and 23 years. Some temperature monitoring sites have been maintained by the Recovery Program Director’s Office (PDO) based in Lakewood, Colorado; others are maintained by the U.S. Fish and Wildlife Service’s (FWS) Grand Junction Fish and Wildlife Conservation Office based in Grand Junction, Colorado. Several of these sites became redundant to temperature monitoring sites installed nearby in 2009 by the U.S. Geological Survey (USGS), and for that reason were discontinued in 2014. The USGS sites report real-time data1. Data from the PDO sites are collected continuously on data loggers, reviewed, and posted to a web site once a year2 . In 2018, two sites were discontinued at the recommendation of the PDO. The two discontinued sites are (1) below Crystal Reservoir and (2) above the North Fork Gunnison River Confluence – highlighted in red in Table 1, and denoted as sites #3 and #4 in Figure 1. Crystal Reservoir, which is just upstream of the first discontinued site, is the most downstream reservoir of the Aspinall Unit. Importance of River Temperatures The Gunnison River downstream of Delta, Colorado is designated critical habitat for two endangered fish species: the Colorado pikeminnow (Ptychocheilus lucius) and the razorback sucker (Xyrauchen texanus). Currently, the observed range of the Colorado pikeminnow in the Gunnison 1 Data can be accessed at https://waterwatch.usgs.gov/wqwatch 2 Data can be accessed at www.fws.gov/mountain‐prairie/riverdata/temperatures.html 1 River extends only about 33 miles upstream of its confluence with the Colorado River, to a point near Delta, Colorado. Cool water temperatures may be a factor limiting the Colorado pikeminnow’s range in the Gunnison River. Osmundson (1999) assessed the potential for further extending the range of the Colorado pikeminnow and determined that river temperatures limit the distribution of this fish in the Gunnison River, in spite of good habitat and prey conditions upstream. His 1999 study indicated that cool water temperatures upstream of the pikeminnow’s current range can interfere with its reproduction and can reduce its growth rate. That study examined the potential for extending the range of the Colorado pikeminnow in the Gunnison River by improving temperature conditions for this warm-water fish. It noted that relatively cool water releases from the reservoirs comprising the Bureau of Reclamation’s Aspinall Unit on the upper Gunnison River cause summer temperatures in the critical habitat to be about 3°C cooler than in other river reaches that have relatively large populations of Colorado pikeminnow. Osmundson proposed that efforts to recover the Colorado pikeminnow seek to increase the temperatures in the Gunnison River at Delta by 1oC in May, June, September and October, and by 2oC in July and August (Boyer and Cutler 2004). Osmundson’s work indicated that increasing mean water temperatures at Delta by these amounts would increase the mean annual thermal units (ATU) from 32 to 46, thus putting stream temperatures at Delta at a level similar to sites on the Yampa and Colorado Rivers which have abundant populations of pikeminnow (Osmundson 1999). Figure 1: Temperature Monitoring Sites in the Gunnison River basin, Colorado 2 Table 1: Recovery Program temperature monitoring sites in the Gunnison River basin, CO. Modeling Opportunities to Improve River Temperatures for Endangered Fish In response to recommendations from Osmundson (1999) to increase the temperatures in designated critical habitat, the Recovery Program sponsored Jean Marie Boyer and Amy Cutler to study alternatives for improving temperatures in the Gunnison River. Their conclusions were published in a 2004 report (Gunnison River / Aspinall Unit Temperature Study – Phase II, Final Report) hereafter referred to as the “Boyer Report”. The Boyer Report evaluated the potential to improve temperature conditions by manipulating instream water temperatures using temperature control devices (TCD’s) at Blue Mesa Reservoir. As part of that work, Boyer and Cutler developed a model to simulate reservoir release water temperatures. They used the CE-QUAL-W2 model version 3.1, developed by the U.S. Army Corps of Engineers, to simulate temperature in Blue Mesa, Morrow Point, and Crystal Reservoirs. The QUAL2K model (Chapra and Pelletier 2003) was used to simulate river temperatures in the Gunnison River from below Crystal Dam to Delta, Colorado. The model results characterize the temperature regimes which are probable over a given series of years, the effect of controlling the reservoir elevations from which water is released (“selective withdrawals”) on the reservoir’s heat budget, and the range of achievable temperatures to meet downstream targets identified by Osmundson (1999). Data provided by the Recovery Program’s monitoring between 1997 and 2001 were used to calibrate the model, including temperature data from the two sites that are the focus of this report, plus a third monitoring site located below the confluence with the Uncompahgre River (site #11 in Figure 1). The Boyer Report assumed for modeling purposes that the total volume released annually from Crystal Reservoir would remain the same, only the timing would change. The study simulated “high-”, “average-”, and “low-flow” scenarios for each month, and evaluated multiple release scenarios: 3 1. Historical releases: Releases corresponded to the 1997-2000 historic conditions, in which flow varied from 0 to 6,015 cubic feet per second (cfs). 2. Flow recommendation releases: Simulated temperatures reflected likely operations if flows proposed by the U.S. Fish and Wildlife Service (McAda 2003) to promote the recovery of the endangered fish in the Gunnison River were adopted. Meeting these recommendations would require higher spring peak flow releases. 3. Releases under multiple temperature control device (TCD) scenarios: Two hypothetical selective withdrawal structures at Blue Mesa Reservoir were modeled: a fixed and a multiple-level TCD. 4. Releases affected by low reservoir storage volumes: This scenario assessed how low storage conditions in Blue Mesa Reservoir (2003 conditions) would affect release temperatures. Figure 2: Temperature releases from the Crystal Reservoir modeled by Boyer and Cutler (2004) under the above scenarios for the years 1997 through 2000. Conclusions from the Boyer Report included: The best location for a temperature control device would be Blue Mesa Dam. Large releases from Crystal Dam combined with increased release temperatures could result in as much a 5o C warming in the Gunnison River at Delta. Smaller releases can result in 1o C warming at Delta and as much as 3o C warming upstream of the North Fork confluence. 4 These increased release temperatures are achievable from June to October with the TCD option. In general, there was not much variation in reservoir stratification and potential release temperatures for the modeled years 1997 through 2001. The PDO’s Two Discontinued Monitoring Sites As noted above, temperature data from the now-discontinued Below Crystal Reservoir and Above North Fork Gunnison River monitoring sites were used by Osmundson (1999) to characterize temperature conditions in the Gunnison River downstream of the Aspinall Unit and by Boyer and Cutler (2004) to calibrate their models and simulate temperature changes likely to occur as the result of specified changes in Aspinall Unit operations. Despite these sites being important for both of these reports, we describe the rationale for discontinuing them below. First Site: Gunnison River below Crystal Reservoir The Gunnison River below Crystal Reservoir temperature site was discontinued in 2018 because data collected at this location for 18 years (1997 through 2014)3 satisfactorily confirms a strong correspondence between the observed temperatures and Boyer and Cutler’s 2004 model predictions, and provides a robust characterization of seasonal temperatures and temperature variations at this location. Furthermore, as no temperature control devices have been installed at Blue Mesa Reservoir, and none are anticipated in the foreseeable future, it does not appear

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