Introduction


Detroit Dam was constructed in 1953 by the U.S. Army Corps of Engineers (USACE) on the North Santiam River in western Oregon and resulted in the formation of Detroit Lake (fig. 1). The North Santiam River drains an area on the western slopes of the Cascade Range, and it is one of several major tributaries to the Willamette River (fig. 2). Detroit Dam is the tallest dam (463 ft) in the Willamette River basin and impounds 455,100 acre-ft of water at full pool, making Detroit Lake one of the largest reservoirs in the basin. The smaller reregulating dam downstream of Detroit Dam, Big Cliff Dam, ensures steady streamflows in the North Santiam River and allows Detroit Dam’s power generating facility (and releases) to be turned on and off during the course of a day to meet peak electrical demands. The Big Cliff–Detroit Dam complex typically generates more hydroelectric power than any other USACE facility in the Willamette River basin, and Detroit Lake ranks as one of the most important recreational resources among the 13 reservoirs managed by USACE in the Willamette Project. 


Prior to 2007, power generation was a high priority for the Big Cliff–Detroit Dam complex, and releases from Detroit Dam generally were routed through the power penstocks (centerline elevation 427.6 m [1,402.9 ft]) except for times when excess flows were released through the upper regulating outlets (ROs, center-line elevation 408.4 m [1,339.9 ft]) or over the spillway (crest elevation 469.7 m [1,541.0 ft]). During those years, midsummer releases were unseasonably cold because the power penstocks are located 166 ft below Detroit Lake’s full-pool level, well below the thermocline at that time of year. Releases from that depth allowed summer solar energy inputs to accumulate in a growing layer of warm water at the lake surface. Drawdown of the lake in September to make room for winter flood storage typically brought that warmer water down to the level of the power penstocks, thus resulting in unseasonably warm releases in late summer and autumn. These somewhat “unnatural” seasonal patterns in the temperature releases can be confusing to anadromous fish, altering the timing of migration, spawning, and egg emergence (Caissie, 2006). The thermal effects of Willamette River basin dams have been quantified in recent modeling studies, and the effects can extend for many miles and many days of travel time downstream (Rounds, 2010).


The North Santiam River and its tributaries (fig. 2) provide habitat for endangered Upper Willamette River Chinook salmon (Oncorhynchus tshawytscha) and Upper Willamette River winter steelhead (O. mykiss). The Oregon Department of Environmental Quality (ODEQ) has set maximum water-temperature standards for stream reaches in Oregon, including the North Santiam and Santiam Rivers, to protect certain life stages of these sensitive fish. These criteria are based on the 7-day moving average of the daily maximum (7dADM) water temperature. For example, the North Santiam River was designated as core cold-water habitat for June 16–August 31 annually, with the 7dADM water temperature not to exceed 16.0°C, and as salmon and steelhead spawning habitat for September 1–June 15, with a stricter 13.0°C criterion. Farther downstream, the Santiam River was designated as salmon and trout rearing and migration habitat with a maximum 7dADM water temperature of 18.0°C for May 16–October 14, and salmon and steelhead spawning habitat for October 15–May 15 with the 13°C maximum criterion for spawning. (Oregon Department of Environmental Quality, 2009).


To protect and enhance these beneficial uses and habitats, the National Marine Fisheries Service wrote a 2008 Willamette Basin Biological Opinion (BiOP) that, among other things, urges the USACE to assess the feasibility of developing project-specific alternatives for achieving long-term temperature control at the Big Cliff–Detroit Dam complex (National Marine Fisheries Service, 2008). The USACE is in the process of evaluating alternatives for both current and long-term downstream temperature management as well as fish passage at many of the dams in the Willamette Project.


Detroit Dam is an excellent facility for the USACE to test strategies for downstream temperature management because the dam has outlets at several fixed elevations, allowing water to be released from multiple depths and blended to meet a downstream temperature target. In particular, the release of warm water over the spillway in midsummer and cool water from deep in the lake in late summer and early autumn can help mitigate problems associated with water temperatures that otherwise are too cold or too warm for fish. Since 2007, USACE has used the spillway and the ROs in addition to the power penstocks to improve downstream fish habitat during the various life stages of endangered salmonid fish species, while at the same time balancing the need to generate hydropower. 


To help evaluate potential dam operation strategies and future structural options, the USACE can rely on predictions from several models of the Detroit Lake and North Santiam River system. The U.S. Geological Survey (USGS) has constructed a model of Detroit Lake to examine water-temperature and suspended-sediment conditions in the lake and downstream (Sullivan and others, 2007). The model was built using CE-QUAL-W2, a two-dimensional, laterally averaged hydrodynamic and water-quality model from USACE (Cole and Wells, 2002) that is widely applied to river and reservoir systems around the world. The USGS Detroit Lake model was calibrated to conditions during calendar years 2002 and 2003 and was tested for high-flow conditions in 2005–06. The model and many results are available online at http://or.water.usgs.gov/santiam/detroit_lake/.


The USGS Detroit Lake model was built with CE-QUAL-W2 version 3.12, modified to include a custom subroutine that allows a model user to easily estimate release rates from different dam outlets that are necessary to achieve a time series of downstream temperature targets (Rounds and Sullivan, 2006; Buccola and Rounds, 2011). In this way, dam operations can be forecast to meet certain downstream fish habitat criteria at different times of the year. CE-QUAL-W2 models of Big Cliff Reservoir and the North Santiam and Santiam Rivers (Sullivan and Rounds, 2004) also are available. Using those models, predicted flows and water temperatures from the Detroit Lake model can be translated downstream to evaluate how temperatures change in the 61 mi of river downstream of Detroit Dam before the Santiam River joins the Willamette River. 


Purpose and Scope


To better inform structural and operational planning decisions related to Detroit Dam outflow temperature management, the USACE asked the USGS to assist in temperature modeling of the Detroit Lake–Big Cliff Reservoir–North Santiam River system. The purpose of this report is to provide water temperature estimates throughout the North Santiam River system from just upstream of Detroit Lake to the junction of the North and South Santiam Rivers (49.2 mi downstream of Detroit Dam) under a range of environmental conditions, alternative dam operations, and potential structural changes at Detroit Dam. Model results presented in this report are intended to inform the current and future operation of Big Cliff and Detroit Dams (and other similar dams in the Pacific Northwest) as well as the planning process for potential structural alterations to Detroit Dam undertaken by USACE for the purpose of improving downstream temperature conditions for fish in the North Santiam River. 


The following guiding objectives were used to examine and quantify the downstream thermal effects of potential operational and structural changes to Detroit Dam: 


• Develop a range of environmental conditions that represent “cool/wet,” “normal,” and “hot/dry” hydrologic and meteorological inputs that can serve as boundary conditions for all scenarios. 

• Estimate water temperatures in the North Santiam River that might occur in the absence of dams. 

• Simulate a range of potential operational and structural scenarios at Detroit Dam and compare predicted outflow temperatures against existing conditions. 

• Simulate conditions downstream of Detroit Dam using the Big Cliff Reservoir and North Santiam River models for a select subset of model scenarios and compare to existing conditions.


This study used previously developed CE-QUAL-W2 models of Detroit Lake (Sullivan and others, 2007), Big Cliff Reservoir (model development documented in appendix A), and the North Santiam River (Sullivan and Rounds, 2004) for all simulations of water discharge and temperature. After an assessment of variations in historical data, measured meteorological and hydrologic data from 2002, 2005, 2006, and 2009 were used in this study for forcing conditions to the models and calculations. The calibration performance of the Detroit Lake and Big Cliff Reservoir models was checked using existing operating conditions in the 2011 calendar year, a year in which measured temperature profiles existed in both lakes. By using measured data from 2002 to 2011, the simulations reflect the most current climatic conditions and take advantage of the extensive datasets collected in recent years.


First posted October 30, 2012

Revised June 11, 2013