Effects of Climate and Dam Operations on Reservoir Thermal Structure

Effects of Climate and Dam Operations on Reservoir Thermal Structure Brett M. Johnson1; Laurel Saito, M.ASCE2; Mark A. Anderson, A.M.ASCE3; Paul Weiss4; Mary Andre5; and Darrell G. Fontane, M.ASCE6 Abstract: Recently, the United States Bureau of Reclamation agreed to increase spring releases from Upper Colorado River Basin reservoirs to create a more natural flow regime in the lower Gunnison River and Upper Colorado River. Fishery managers have expressed concern that new operations could change reservoir conditions and jeopardize popular and economically important reservoir sport fisheries. This study attempts to predict how one aspect of reservoir conditions, thermal structure, might respond to new dam operations at Blue Mesa Reservoir aimed at addressing downstream ecological concerns. A one-dimensional thermal model ͑CE-THERM͒ is applied to simulating thermal effects of ‘‘traditional’’ and proposed ‘‘new’’ dam operation scenarios. To evaluate the relative importance of climate and dam operations a sensitivity analysis of hydrologic ͑i.e., inflows and starting reservoir elevation͒ and meteorologic ͑i.e., air temperature, cloud cover, and dew point temperature͒ inputs was conducted along with an ‘‘extreme’’ dam operation scenario. Results indicate that reservoir managers at Blue Mesa Reservoir have considerable latitude for new operations without negative thermal consequences. The natural variability of climate and hydrology appear to exert stronger control over reservoir thermal structure than reservoir operations at Blue Mesa. DOI: 10.1061/͑ASCE͒0733-9496͑2004͒130:2͑112͒ CE Database subject headings: Dams; Reservoirs; Simulation models; Climatic changes; Colorado River; Thermal factors. Introduction Recently, river ecologists have advocated the use of new dam operations to create a more natural flow regime for restoration of In addition to their intended purposes, dams have a variety of regulated rivers ͑National Research Council ͑NRC͒ 1991; Stan- physical, chemical, and biological effects on the rivers they im- ford et al. 1996; Poff et al. 1997͒. One important element of a pound ͑Baxter 1977; Ward and Stanford 1979; Collier et al. natural flow regime that is most often constrained by dams is the 1996͒. Increasing public concern over adverse, downstream effects of dams has prompted debate about the efficacy of dam amplitude of annual peak flows. In free-flowing rivers, these peak removal for regulated river restoration. However, many dams now flows are part of the natural channel maintenance process, and provide indispensable services to society such as water supply, they maintain connections between the channel and its floodplain hydropower, and flood control. Efforts to mitigate the ecological habitats. Natural variation in flow also plays an important role in ͑ ͒ effects of many dams will focus, at least in the short term, on the life cycle processes of stream biota Allan 1995 . changing dam operations to minimize riverine impacts. To achieve a more natural flow regime downstream, dam op- erators must depart from their historic operating regimes. In the 1 Rocky Mountain West, where natural hydrographs are character- Associate Professor, Dept. of Fishery and Wildlife Biology, ͑ ͒ Colorado State Univ., Fort Collins, CO 80523-1474. E-mail: ized by a spring snowmelt peak Van Steeter and Pitlick 1998 , [email protected] dams have traditionally been used to capture high spring inflows 2Assistant Professor, Dept. of Natural Resources and Environmental for consumptive uses during drier summer months. Operations to Science/186, Univ. of Nevada-Reno, 1000 Valley Rd., Reno, NV 89512. achieve this goal are constrained by other needs, such as the need E-mail: [email protected] to maintain adequate storage capacity for flood control, to main- 3 Associate Engineer, CH2M Hill, 825 NE Multnomah, Suite 1300, tain minimum downstream flows, to reach elevation targets for Portland, OR 97232-2146. E-mail: [email protected] recreational and aesthetic purposes, and to generate hydropower 4Engineer, Riverside Technology, 2209 E. Prospect Rd., Suite l, Fort during periods of increased demand. Many large western dams Collins, CO 80525. E-mail: [email protected] 5Project Engineer, Civil Design Consultants, Inc., P.O. Box 775167, were constructed with hypolimnetic outlets to facilitate hydro- 405 S. Lincoln Ave., Steamboat Springs, CO 80477-5167. E-mail: power generation and maximize water available for release. [email protected] New dam operations on the Colorado River system have re- 6Professor, Dept. of Civil Engineering, Colorado State Univ., Fort cently focused on two areas: the construction of selective with- Collins, CO 80523-1372. E-mail: [email protected] drawal devices for temperature control ͑e.g., at Flaming Gorge Note. Discussion open until August 1, 2004. Separate discussions Dam, Utah; and Glen Canyon Dam, Arizona͒ and altered release must be submitted for individual papers. To extend the closing date by schedules to manage the downstream hydrograph ͑e.g., Blue one month, a written request must be filed with the ASCE Managing Mesa Dam, Colorado, as well as at Flaming Gorge and Glen Editor. The manuscript for this paper was submitted for review and pos- ͒ sible publication on March 8, 2002; approved on April 2, 2003. This Canyon Dams . Historic operations of dams on the Colorado paper is part of the Journal of Water Resources Planning and Manage- River system appear to have had adverse effects on rare and en- ment, Vol. 130, No. 2, March 1, 2004. ©ASCE, ISSN 0733-9496/2004/2- dangered fishes by creating hydrologic conditions unfavorable for 112–122/$18.00. spawning or recruitment ͑Tyus 1991; Stanford 1994; Osmundson 112 / JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT © ASCE / MARCH/APRIL 2004 and Burnham 1998͒. In recent consultations between the United States Fish and Wildlife Service ͑USFWS͒ and the United States Bureau of Reclamation ͑USBR͒ under Section 7 of the Endan- gered Species Act, the USFWS Upper Colorado River Endan- gered Fish Recovery Program requested new dam operations that would create a more natural flow regime in the lower Gunnison River and Upper Colorado River ͑McAda 2003͒. To achieve this, USBR agreed to implement increased spring releases from Upper Colorado River Basin reservoirs. However, little was known about potential in-reservoir impacts of increased spring releases from western reservoirs, and reservoir fishery managers expressed concern that new operations could jeopardize popular and economically important reservoir sport fisheries. Changing the schedule of releases could alter seasonal reservoir elevations and reservoir thermal structure ͑Thornton et al. 1990͒. Changes to thermal structure can have far-reaching direct ͑on physiological rates of organisms͒ and indirect ͑on water col- umn stability and nutrient cycling, and prey production͒ consequences for system productivity ͑Wetzel 2001͒. Studies on Lake Granby, a 3,000 ha montane reservoir near the headwaters of the Colorado River, show a strong correlation between reservoir con- tent ͑volume of water in the reservoir͒ in July and the thermal structure of the reservoir ͑Martinez and Wiltzius 1995͒. Changes in thermal structure have several important biological consequences affecting the distribution of biota in the reservoir, zoop- lankton production, and sport fish growth rates. However, that study failed to reveal the extent to which reservoir operations versus weather and hydrologic variables contribute to the ob- served patterns in thermal structure and biological indicators. This study attempts to predict how reservoir thermal structure might respond to new dam operations at Blue Mesa Reservoir that were aimed at addressing downstream concerns. The range of potential dam operations is constrained by a variety of socioeco- nomic factors that may be in conflict with river restoration goals. Fig. 1. ͑A͒ Location and configuration of Blue Mesa Reservoir. Dam operation regimes that are likely to occur are simulated and Sampling stations ͑1–3͒ are also indicated. ͑B͒ Schematic of Blue compared to predictions from an extreme scenario. Since reser- Mesa Dam and reservoir in cross section ͑USBR 1975͒. voir heat budgets are sensitive to climatic and hydrologic effects, the relative sensitivity of reservoir thermal structure to variations in dam operations and climate/hydrologic factors is also evalu- ated. Ultimately, the interest is in evaluating the degree to which the annual inflow occurs in spring in a pattern characteristic of a ͑ ͒ human-induced i.e., dam operations factors alter reservoir ther- snowmelt hydrograph. The regional climate is semi-arid with mal structure, and thus the likelihood that changing dam opera- about 260 mm of precipitation annually. tions might indirectly alter biological conditions in the reservoir. Description of Thermal Model Study Area: Blue Mesa Reservoir CE-THERM is a one-dimensional model of reservoir thermal Blue Mesa Reservoir is a 25 km long, 3,700 ha impoundment on structure. The model is an independent program within CE- the Gunnison River in southwest Colorado ͓Fig. 1͑A͔͒. The Gun- QUAL-R1, a reservoir water quality model developed by the U.S. nison River flows into the Colorado River at Grand Junction, Army Corps of Engineers ͑USACE͒ at the Engineering Research Colorado. As part of the USBR’s Wayne N. Aspinall Unit of the and Development Center and originally released in the 1970s. The Colorado River Storage Project, Blue Mesa Reservoir serves model simulates reservoir thermal dynamics over a series

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