- Lane County)
(From Atlas of Oregon Lakes, Johnson et al. 1985). Dexter Lake is a relatively small, shallow reservoir which functions as a reregulating basin for Lookout Point Lake immediately upstream. As such it is designed to contain, temporarily, surges of water released intermittently from Lookout Point Lake during hydroelectric power generation, thereby avoiding widely fluctuating flows downstream. Accordingly, the volume of water stored in Dexter Lake varies considerably throughout the year. It is one of 13 multi-purpose water projects built by the Corps of Engineers in the Willamette Valley. Dexter and Lookout Point Lakes were completed in 1954 with the construction of dams on the Middle Fork of the Willamette River and, in combination, have since prevented an estimated $200 million in damages from downstream flooding. A small amount of hydroelectric power is also generated from a powerhouse at Dexter Dam with one generating unit. In spite of fluctuations in surface water level, Dexter Lake is a very popular recreation site, used very heavily by water-skiers and sailboaters. Lane County parks on both the north and south shores provide full day-use facilities. Angling is also a popular activity and the lake is open for fishing all year, although there have been problems with infestation by rough fish in recent years.
The drainage basin for Dexter Lake is, of course, the same as for Lookout Point Lake; it is a forested area stretching to the crest of the Cascade Mountains. The lake itself lies in the transition zone between the mountains and the Willamette Valley, an area of gentle topography that consists of alluvial fill and old river terraces, surrounded by remnant buttes of basalt and diorite. The western hemlock and Douglas fir forests that dominate the drainage basin show great variety around the lake where there has been significant alteration of vegetation. About three-quarters of the drainage basin is managed by the Willamette National Forest. There are significant amounts of private holdings upstream around the town of Oakridge and adjacent to the lower half of Lookout Point Lake and around Dexter Lake.
The thermal properties of this reservoir are somewhat unusual because it is fed by cool water discharged from the bottom of Lookout Point Lake. This cool water tends to sink in Dexter Lake and maintains thermal stratification which would otherwise not develop in such a shallow reservoir. The major ion chemistry is typical of western Cascade reservoirs. There is extensive growth of rooted macrophytes and occasional algal blooms, including diatoms and cyanophytes. Chlorophyl and phosphorous concentrations and water transparency suggest that the lake is mesotrophic. However, because the lake shows some tendency to develop algal blooms and oxygen is sometimes depleted, it is classified as mesotrophic, a condition encouraged by the shallow depth.