Effects of Climate and Dam Operations on Reservoir Thermal Structure Brett M. Johnson 1 ; Laurel Saito, M.ASCE 2 ; Mark A. Anderson, A.M.ASCE 3 ; Paul Weiss 4 ; Mary Andre 5 ; and Darrell G. Fontane, M.ASCE 6 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-THERM 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 elevation and meteorologic i.e., air tempera- ture, cloud cover, and dew point temperature 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/ASCE0733-94962004130:2112 CE Database subject headings: Dams; Reservoirs; Simulation models; Climatic changes; Colorado River; Thermal factors. Introduction In addition to their intended purposes, dams have a variety of physical, chemical, and biological effects on the rivers they im- pound Baxter 1977; Ward and Stanford 1979; Collier et al. 1996. Increasing public concern over adverse, downstream ef- fects of dams has prompted debate about the efficacy of dam removal for regulated river restoration. However, many dams now provide indispensable services to society such as water supply, hydropower, and flood control. Efforts to mitigate the ecological effects of many dams will focus, at least in the short term, on changing dam operations to minimize riverine impacts. Recently, river ecologists have advocated the use of new dam operations to create a more natural flow regime for restoration of regulated rivers National Research Council NRC 1991; Stan- ford et al. 1996; Poff et al. 1997. One important element of a natural flow regime that is most often constrained by dams is the amplitude of annual peak flows. In free-flowing rivers, these peak flows are part of the natural channel maintenance process, and they maintain connections between the channel and its floodplain habitats. Natural variation in flow also plays an important role in the life cycle processes of stream biota Allan 1995. To achieve a more natural flow regime downstream, dam op- erators must depart from their historic operating regimes. In the Rocky Mountain West, where natural hydrographs are character- ized by a spring snowmelt peak Van Steeter and Pitlick 1998, dams have traditionally been used to capture high spring inflows for consumptive uses during drier summer months. Operations to achieve this goal are constrained by other needs, such as the need to maintain adequate storage capacity for flood control, to main- tain minimum downstream flows, to reach elevation targets for recreational and aesthetic purposes, and to generate hydropower during periods of increased demand. Many large western dams were constructed with hypolimnetic outlets to facilitate hydro- power generation and maximize water available for release. New dam operations on the Colorado River system have re- cently focused on two areas: the construction of selective with- drawal devices for temperature control e.g., at Flaming Gorge Dam, Utah; and Glen Canyon Dam, Arizona and altered release schedules to manage the downstream hydrograph e.g., Blue Mesa Dam, Colorado, as well as at Flaming Gorge and Glen Canyon Dams. Historic operations of dams on the Colorado River system appear to have had adverse effects on rare and en- dangered fishes by creating hydrologic conditions unfavorable for spawning or recruitment Tyus 1991; Stanford 1994; Osmundson 1 Associate Professor, Dept. of Fishery and Wildlife Biology, Colorado State Univ., Fort Collins, CO 80523-1474. E-mail: brett@cnr.colostate.edu 2 Assistant Professor, Dept. of Natural Resources and Environmental Science/186, Univ. of Nevada-Reno, 1000 Valley Rd., Reno, NV 89512. E-mail: lsaito@cabnr.unr.edu 3 Associate Engineer, CH2M Hill, 825 NE Multnomah, Suite 1300, Portland, OR 97232-2146. E-mail: manders4@ch2m.com 4 Engineer, Riverside Technology, 2209 E. Prospect Rd., Suite l, Fort Collins, CO 80525. E-mail: psw@riverside.com 5 Project Engineer, Civil Design Consultants, Inc., P.O. Box 775167, 405 S. Lincoln Ave., Steamboat Springs, CO 80477-5167. E-mail: andre@bwn.net 6 Professor, Dept. of Civil Engineering, Colorado State Univ., Fort Collins, CO 80523-1372. E-mail: fontane@engr.colostate.edu Note. Discussion open until August 1, 2004. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and pos- sible publication on March 8, 2002; approved on April 2, 2003. This paper is part of the Journal of Water Resources Planning and Manage- ment, Vol. 130, No. 2, March 1, 2004. ©ASCE, ISSN 0733-9496/2004/2- 112–122/$18.00. 112 / JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT © ASCE / MARCH/APRIL 2004