Detroit Lake (Linn, Marion)

Reachcode: 17090005012370 | Area: 3591.4 acres | Shoreline: 38.3 mi | View on Interactive Map

(From Atlas of Oregon Lakes, Johnson et al. 1985).  Detroit Lake is a large storage reservoir on the North Fork of the Santiam River, one of 13 multipurpose, water resource projects in the Willamette River basin constructed and operated by the U. S. Army Corps of Engineers. The project became operational in 1953, at the same time as Big Cliff Lake, a reregulating reservoir (surge basin) immediately downstream from Detroit Lake. Detroit Lake was intended primarily for flood control and power generation, but the large reservoir has become one of the major recreation resources in western Oregon. Nearly half a million people visit each year, most of them coming during the busy summer months when high water levels are maintained. It is a beautiful, nine-mile long impoundment which twists its way through the western slopes of the Cascade Mountains below Mt. Jefferson. Three main arms branch off the main stem, and the flooded tributaries reach deeply into the mountainous and forested terrain. Shoreline slopes are quite steep and, as a result, very little land around the lake is suited for development. There are several patches of private land on these slopes. The town of Detroit, from which Detroit Lake derives its name (many Michigan people have lived in the area), lies on the shoreline where the Breitenbush River joins the lake.

The lower portion of the drainage basin consists of andesite lava flows mixed with ash beds, all deeply incised and dissected by stream action. Some slope failure has occurred in parts of the basin but turbidity has not been a problem as in some other area reservoirs (for example, Hills Creek Lake). A granitic intrusion, a fairly uncommon rock type for this part of the state, crops up by Detroit Dam. It is a mass of andesitic magma that did not reach the surface and cooled more slowly at depth. Possibly it represents the core of an old volcano that fed the surrounding volcanoes during the buildup of the Cascades about 25 million years ago. Most of the large drainage basin is covered with a coniferous forest managed by the Willamette National Forest. Douglas fir is the dominant species in the lower portion of the drainage basin, grading into hemlock and true firs at the higher elevations.

Fishing leads the list of recreational activities at Detroit Lake, with an estimated 100,000 angling-days and a catch of 200,000 fish per year. About 85 percent of the catch is rainbow trout; these are stocked in large numbers by the Oregon Department of Fish and Wildlife, with legal-size and fingerlings added monthly in summer. Piety Knob, a large island rising 450 feet from the center of the reservoir, is adjacent to a favorite trolling area. Brook trout, cutthroat trout, kokanee, and bullhead supplement the trout fishery, and chinook salmon have occasionally been stocked. Since the 463-foot high concrete dam presents an effective barrier to migrating adult salmon and steelhead, these fish are intercepted six miles downstream at Minto Station. Eggs are artificially fertilized here and taken upstream to Marion Forks Hatchery, where the fingerlings are released. Unfortunately, the Minto facility has failed to attract sufficient numbers of chinook because of low-temperature hydropower releases from Detroit Lake which tend to hold water temperatures well below the optional temperature range for chinook

migration (Larson 1980). Detroit Lake also receives some of the heaviest pleasure boating use in the state. Based on the total number of boating days estimated for 1975, this was the third "most used" lake in the state, following coastal Tenmile Lake and Diamond Lake (Frenkel 1975). Five boat ramps and two marinas help make it a very popular water-skiing spot. One campground, located on Piety Knob, can be reached only by boat. Three other campgrounds, including the large Detroit State Park on the north shore, provide over 200 tent and trailer spots around the lake. Two marinas in town offer boat rentals, moorage, and other accommodations.

The water level in Detroit Lake varies throughout the year, a pattern typical of other storage impoundments in the Willamette Valley. It is a response to seasonal patterns of precipitation and'runoff, and also to downstream requirements and power demands. From 1 December until 31 January, which is the major flood period, the volume of water is about one-half the maximum to allow for storage of flood waters. During late winter and spring, surface runoff fills the reservoir to near capacity, except during drier years. Until August, it is kept at a fairly constant high level, coincident with the recreation season. August through November is the season during which the water is gradually drawn back down to the minimum flood control elevation. This minimum level is 100 feet below full pool elevation and the surface area of the lake is about one-half of the area at full pool. In fall, the released water is used primarily for power production, and two generator units provide 100 megawatts of peak electrical output. 

Detroit Lake develops a pronounced thermal stratification with surface water as warm as 68 degrees Fahrenheit (20 degrees Celsius) in the summer, whereas water temperature below 100 feet does not exceed 50 degrees Fahrenheit (10 degrees Celsius). Major ion concentrations arelow, as is characteristic of Cascade reservoirs, and oxygen is near saturation at all depths. Chlorophyl concentrations and water transparency indicate that the lake is mesotrophic or moderately productive. The concentration of phosphorus suggests that the lake is eutrophic; however, the lake is quite deep and the phosphorus in the lake evidently does not stimulate eutrophic conditions. Overall, Detroit Lake is mesotrophic, although at the low end. There is no evidence of cultural enrichment from the high amount of recreational use. The phytoplankton is composed primarily of four algal species. Rhodomonas minuta was always dominant in the samples collected in 1982; its occurrence is not associated with any particular trophic state. Fragilaria crotonensis, and to a lesser extent Anabaena, are common, and both indicate mesotrophic to eutrophic conditions. Cryptomonas erosa does not indicate any trophic state, as it is equally common in oligotrophic to eutrophic lakes. The total phytoplankton densities are typical of mesotrophic lakes.