Bull Run Reservoir Number One (Multnomah)
Reachcode: 17080001017566 | Area: 416.3 acres | Shoreline: 9.5 mi | View on Interactive Map
(From Atlas of Oregon Lakes, Johnson et al. 1985). Bull Run Reservoir #1, also known as Lake Ben Morrow, was built on the Bull Run River in 1929 to supplement the water supply for the city of Portland. It lies near the center of the Bull Run Reserve, a restricted area of 214 square miles that provides a reliable supply of clean water for nearly one-third of all Oregonians. To safeguard water quality, the entire reserve is closed to the general public. Hydroelectric generation facilities were installed at the reservoir site in 1982 by Portland General Electric.
The Bull Run Watershed itself occupies about 50 percent of the Reserve. It is a geologically recent area, underlain by alternating flows of andesite and basalt. Larch Mountain, a beautiful example of a shield volcano built from andesite, rises to the north of the reservoir. The fractured lava flows are effective in storing snowmelt runoff and maintaining a relatively high and steady streamflow in the Bull Run River during the dry summer months. Glaciation during the Pleistocene Epoch played a major role in reshaping this volcanic landscape, and the reservoir is set in an area of loess and glacial till. Slopes around the reservoir are generally steep, and post-glacial stream incision has carved some steep canyons in the headwater portions of the drainage basin.
Water in the reservoir is of very high quality, with low conductivity and low alkalinity. Water transparency is also high. Species of diatoms suggestive of mesotrophic conditions have been observed in the water, but populations are low and Bull Run Reservoir #1 is classified as oligotrophic. The water column develops a sharp thermal stratification with water below 30 feet much cooler than surface water during the summer. The only impediment to maintaining the desired high level of water quality in the reservoir is an occasional problem with turbidity. A major portion of the land within the Bull Run Watershed contains relatively stable soils formed from basalt and andesite parent materials. However, soils of silt and clay composition are found in the upper layers sandwiched between the lava flows and are actively being eroded. Although these unstable soils occupy only 1.5 percent of the area, they constitute the greatest hazard for increased turbidity because they are found adjacent to stream channels. The North and South Forks of Bull Run River have cut into these unstable materials, and the turbidity of these streams is noticeably greater than other streams whose channels are formed in harder rock.