Roslyn Lake (Clackamas)

Reachcode: 17080001017591 | Area: 144.3 acres | Shoreline: 2.8 mi | View on Interactive Map

Roslyn Lake was a historic reservoir built by Portland General Electric for the Bull Run Hydroelectric Project.  The dam that formed the lake was removed in 2008 as part of the decommissioning of the hydroelectric project. 

(From Atlas of Oregon Lakes, Johnson et al. 1985)  Roslyn Lake is an artificial lake built by Portland General Electric as a storage reservoir for the Bull Run power plant. Primary inflow is diverted from the Sandy River from a point about five miles upstream and carried to the lake by tunnel and flume. Excess water from Portland's Bull Run water supply is also periodically diverted into the lake. Water is released through an outlet structure into the power house, and discharged from there into the Sandy River. The lake was formed by the construction of a stone and earth dike almost entirely around it, so that the water level is in most places above the level of the surrounding countryside. Years ago, before the power canal was screened, trout and steelhead came into the lake, and many were caught as there was no way out for the fish. Fish can no longer come into the lake, but rainbow trout are stocked by the Oregon Department of Fish and Wildlife. Best angling is reported to be on the east side where the inflow is located. No motorboats are allowed on the lake. A P.G.E. recreational area along the southeast side of the lake has excellent picnic facilities and is used frequently by visitors from the Portland metropolitan area.

The ecological characteristics of Roslyn Lake are influenced by two factors -- its shallow depth and the inflow from the Bull Run and Sandy Rivers. Wave action stirring the sediments, in addition to contributing nutrients to the lake, causes the water to become muddy at times. The very transparent, low-nutrient inflow tends to counteract these effects by diluting the lake water and thereby lowering suspended particulate and nutrient concentrations. Secchi disk measurements indicate highly mesotrophic conditions. Chlorophyl, phosphorus, and major ion concentrations are moderate, and also indicate mesotrophic conditions for Roslyn Lake. The bottom of the lake consists of mud and sand, and supports several species of macrophytes (Elodea canadensis, Myriophyllum exalbescens, Lemna minor, and others).

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