09/20/2026
From Atlas of Oregon Lakes, Johnson et al. 1985) Foster Lake is one of 13 multi-purpose reservoirs in the Willamette Valley built and operated by the U. S. Army Corps of Engineers. Construction began on Foster Dam and on Green Peter Dam, six miles upstream, in 1961 and was completed in 1968. Foster Dam is immediately below the confluence of the Middle Santiam and South Santiam Rivers, and impounds waters from both. The dam and lake are named for the nearby town of Foster, in turn named for G. P. Foster, who operated a grist mill nearby early in this century.
This wide reservoir is used to reregulate, or "smooth out", flows from Green Peter Reservoir as well as to provide storage for flood control, irrigation, and power production. The high visibility of Foster Lake, its nearness to urban centers, and the limited number of other recreational facilities nearby combine to make it a popular recreation site. Summer boating pressure can be heavy, and water skiing is a favorite sport. Rainbow trout are annually stocked, and angling has been good, with some fair-sized fish taken each year. A paved road leads around the lake to several Linn County parks and boat ramps, and a campground is maintained on the eastern shore.
Foster Lake is set in the transition zone between the rolling low hills of the Willamette Valley and the rugged slopes of the Western Cascades. Two narrow arms of the impoundment reach eastward into the steep canyons of the primary inflowing streams. The project lies on the lower edge of the western hemlock vegetation zone, but a history of logging in the drainage basin has altered the dominant species to Douglas fir. Agriculture, roads, and urban land uses have further changed the natural setting around the lake. Although the Corps administers the shoreline, most of the lower drainage basin and land surrounding the lake is privately owned.
Water level is lowered each fall beginning at the end of September for flood control purposes, reaching minimum level by mid-November. The reservoir is refilled each year beginning in February and reaching full pool in mid-May. The surface level fluctuates daily by two feet or less as power demand requires. Power generation at Foster Dam is primarily for base load, and water level fluctuations result from the intermittent discharge from Green Peter Reservoir (upstream) associated with peak power demands. There are some shoreline areas where bank erosion produces local turbidity, as is commonly observed in recently constructed reservoirs. In time, these areas will stabilize and further erosion will diminish. In the winter, there is also significant turbidity during periods of heavy runoff. The chemistry of the water in Foster Lake is typical of Willamette Valley reservoirs. Major ions are present in low concentration and the alkalinity and conductivity are low. The lake develops a distinct temperature stratification in the summer. As in several other reservoirs, surface water temperatures are high enough to interfere with the trout survival in summer. However, a fishery based on planted trout has been successful. Some oxygen depletion of the bottom water occurs, which may also interfere with fish survival. Water transparency is good in the summer, and chlorophyl and phosphorus are rather low, indicating mesotrophic conditions (but close to oligotrophic).
