Scientific Investigations Report 2007–5008

Scientific Investigations Report 2007–5008

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The U.S. Army Corps of Engineers (USACE) constructed and operates a system of 13 dams and reservoirs in Oregon’s Willamette Basin; Detroit Lake is one of the larger of these reservoirs on the western slope of the Cascade Range. The lake is situated on the North Santiam River (fig. 1) behind Detroit Dam, a 141-m high concrete structure finished in 1953. The watershed encompasses 1,130 km2, and at a full-pool water-surface elevation of 478.2 m, 561 million m3 (455,000 acre-ft) of water is stored in the reservoir, with a surface area of 14.5 km2. The water-surface elevation varies substantially, by 35 m or more, over the course of a year. It is kept high in summer for recreation, and drawn down in winter for flood control. Detroit Lake also is used for power generation, irrigation, and improvement of downstream navigation.

A smaller, reregulating dam, Big Cliff, lies 5 km downstream of Detroit Dam. It is designed to dampen the flow variations caused by Detroit Dam’s power generating operations. There are an additional 36 km2 of watershed area between Detroit and Big Cliff Dams. At a full-pool water-surface elevation of 367.7 m, Big Cliff Reservoir holds 8 million m3 (6,450 acre-ft of water), with a surface area of 0.6 km2.

Major inflows to Detroit Lake include the North Santiam and Breitenbush Rivers, and French, Blowout, Box Canyon, and Kinney Creeks. The watershed reaches its highest point at Mount Jefferson at 3,204 m. The climate of this area is a temperate one, characterized by dry summers and wet winters. Mean annual precipitation for 1971–2000 was 228 cm, and most fell between November and April (Taylor, 2002). For the same period, the average annual air temperature at Detroit Dam was 10.6 ºC. The coldest month was January, with an average temperature of 3.7 ºC, and the warmest month was August, with an average temperature of 18.8 ºC. The watershed geology is dominated by volcanic andesites and basalts with some alluvial and glacial deposits (Uhrich and Bragg, 2003). Steep slopes and weathered, clay-rich soils lead to landslides and earthflows that deliver sediment into many of the creeks and rivers of the upper basin. The land is mostly forested, and timber harvesting and recreation are the largest land uses. More than 95 percent of the land in the watershed upstream of Detroit Dam is part of the Willamette National Forest, with the rest divided between the Mount Hood National Forest, private owners, the Oregon Department of Forestry (ODF), the Warm Springs Indian Reservation, and the Bureau of Land Management.

Downstream of Detroit and Big Cliff Dams, the North Santiam River flows 75 km to the Santiam River, which flows 20 km to the Willamette River between the cities of Albany and Salem. The North Santiam River is governed by Oregon’s “Three Basin Rule” (Oregon Department of Environmental Quality, 2003), designed to protect the river’s water quality, particularly as a source of clean drinking water. The rule has been successful at limiting new point source discharges into the river. Water temperature and suspended sediment, however, remain issues of concern.

Portions of the North Santiam and Santiam Rivers downstream of Detroit Lake exceed Oregon’s maximum water temperature criteria at times, and these reaches were included on Oregon’s most recent 303(d) list of impaired waterbodies (Oregon Department of Environmental Quality, 2006). To address this issue, a water temperature TMDL (Total Maximum Daily Load) was developed for these and other rivers in the Willamette Basin; the TMDL was signed by Oregon’s Department of Environmental Quality on September 21, 2006. As part of that work, a hydrodynamic and water temperature model was developed, and simulations were run, to examine the effects of riparian shading, weather conditions, point sources, and Detroit Dam release flows and temperatures on North Santiam River water temperatures (Sullivan and Rounds, 2004). That study showed that water temperatures in these rivers were indeed sensitive to the temperatures and flows released from Detroit Dam, as well as some of the other factors. Although State temperature criteria focus on problems related to water that is too warm, temperatures also can be too cool for fish at times. Populations of spring Chinook salmon (Oncorhynchus tshawytscha) are declining in the North Santiam River, and dam outflow temperatures that are too cool in midsummer may be a contributing factor (E&S Environmental Chemistry, Inc. and North Santiam Watershed Council, 2002). The outflow of Detroit Lake in June generally is 5–7 ºC, cooler than the ideal of 10–13 ºC for migrating adult Chinook (Larson, 2000).

Suspended sediment concentrations in the North Santiam River also can be of concern. The City of Salem takes its primary supply of drinking water from the North Santiam River approximately 45 km downstream of Detroit Lake. In the past, during some large storms and periods of sustained high levels of turbidity in the lake, high concentrations of suspended sediment in the North Santiam River have required the city to shut off its water intake and, later, to construct a chemical pretreatment system. The U.S. Geological Survey (USGS) has worked in partnership with the City of Salem since 1998 to monitor and study sediment and turbidity throughout the North Santiam River watershed (Uhrich and Bragg, 2003; Bragg and Uhrich, 2004).

Purpose and Scope

Developing a model that simulates the transport and fate of suspended sediment and the dynamics of water temperature in Detroit Lake is an important component of understanding how the lake affects suspended sediment and temperature in the North Santiam and Santiam Rivers downstream of Detroit Lake. The objectives of this study were to (1) develop a model of Detroit Lake to simulate circulation, water temperature, TDS, and suspended sediment in the reservoir and the reservoir’s outflow, (2) understand processes affecting suspended sediment and quantify sediment sources and transport to the lake outlet as well as deposition in the lake, and (3) understand processes controlling water temperature in the lake and lake outflow, and demonstrate the water temperature effects of a hypothetical selective withdrawal device.

The Detroit Lake model, described in this report, was developed for the entire calendar years of 2002 and 2003 and also for the period December 1, 2005, through February 1, 2006 (the “2005–06 storms”) in order to simulate some large winter storm events. During January 2006, about 70 cm (27.6 in.) of precipitation were recorded at Detroit Dam, making it the wettest January ever recorded, breaking the previous record set in 1970 (Oregon Climate Service, 2006). Processes occurring in Big Cliff Reservoir, the reregulating reservoir downstream of Detroit Lake, were not included in this model. The monitoring station on the North Santiam River at Niagara is downstream of Big Cliff Dam, and outflows from Detroit Lake could be influenced by processes in the 6 km reach between the outflow and Niagara, including heat exchange across the air-water interface, and tributary inflows.

This investigation resulted from a scientific and financial partnership between the USGS and the City of Salem, Oregon. Funding for the installation of a meteorological station near Detroit Lake was provided by USACE.

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