- Clackamas County)
(From Atlas of Oregon Lakes, Johnson et al. 1985). Lake Oswego, located in the heart of the city of the same name, is a well-known but difficult-to-see lake in the Portland metropolitan area. The entire shoreline is private land and rimmed with residential neighborhoods, and the lake is visible only in glimpses from the roads surrounding it. The lakebed is owned and maintained by the Lake Oswego Shorefront Committee, a corporation whose members have lake frontage or access rights to the water and recreational facilities through property agreements. Even with this restricted access, Lake Oswego receives very heavy recreational use. According to Frenkel (1975), it is the seventh busiest body of water in Oregon, and perhaps the busiest for use by water-skiers.
Lake Oswego has an interesting geologic history. It occupies a former channel of the Tualatin River, carved in Columbia River basalt. Downcutting and excavation eventually captured the main channel of the Tualatin River, leaving the old Lake Oswego route abandoned. About 13,000 years ago flood waters from glacial Lake Missoula raced down the Columbia River, backed up the Willamette River, and poured through the Lake Oswego gap, deeply scouring and enlarging the channel.
The cultural history of the lake is equally as interesting. It was called Waluga ("Wild Swan") by the Indians, while early settlers gave it the less appealing name of Sucker Lake. Discovery of iron ore beds brought the first iron furnace west of the Rockies, built just below the lake in 1805 at the point where Oswego Creek pours into the Willamette River. Although the area did not live up to initial predictions that it would become the "Pittsburgh of the West", numerous summer cottages and then permanent homes were built on attractive lake front property. In 1913 the name of the lake was changed to Lake Oswego for the community that had grown up on its shores, in turn named for pioneer Walter Durham's hometown of Oswego, New York. Early in the century a canal connected with the Tualatin River and a small dam on the outlet increased the water level of the lake by several feet. The purpose of the canal was to provide a commercial route for flat-bottomed wheat freighters and rafts of logs. Through neglect, the canal filled with grass and willows, impeding its use. In 1939 a project to clear and widen the canal begun, but work was interrupted by World War II; it was completed in 1948. In February, 1941, the Oregon Iron and Steel Corporation gave the lake bed to the Lake Oswego Shorefront Committee, but has retained control of the dam and power plant. Once a year the lake is lowered about six feet for a three week period so that owners may repair their seawalls and docks.
The water quality in Lake Oswego is poor and by all indications it is a hypereutrophic lake. When sampled on 9/21/82, the hypolimnion was anoxic (oxygen-depleted) below 22 feet (7 meters), the transparency was low (Secchi disk depth = 4.6 feet; 1.4 meters), and the concentrations of total phosphorus and phytoplankton were very high, as were nitrogen values. Conductivity, major ion concentrations, and pH were also quite high (although calcium was relatively low). These are all characteristics of a hypereutrophic lake. Nutrient sources into the water include the Tualatin River (the major inflow), activities by homeowners (lawn fertilizers, insect sprays, etc.), non-point sources from within the extensively developed drainage basin, and from waterfowl on the lake. Also, the anoxic sediments probably release phosphorus.
Macrophytes are not a problem. In fact, very few macrophytes can be found in the littoral areas of the lake, probably a result of the annual lake draw-down. Phytoplankton blooms are common, and the City of Lake Oswego treats these blooms with copper sulfate as they start to develop. However, several residents and lake users have indicated that these algal blooms are not bothersome. The phytoplankton density observed on 9/21/82 (3696 algae per ml.) was among the highest from any Oregon lake. More unusual, however, is the species composition of the phytoplankton. With the exception of Rhodomonas minuta which is widespread, the dominant algae were not found in any other lake sampled in this state-wide inventory, even at low densities. It may be that the annual additions of copper have a significant effect upon the phytoplankton species, with only copper tolerant species able to survive.
There is very little existing biological and chemical data on Lake Oswego, partially because it is a private lake without public access. Future studies on Lake Oswego could be mutually beneficial for the scientific community as well as for city residents. The present management of the lake includes two treatments that would be of interest to limnologists: lake draw-down, and copper additions to the water. Monitoring lake conditions before, during, and after these treatments could provide insight into basic limnological interactions under actual field conditions, while the generated data would also benefit the City of Lake Oswego by providing information that may reduce the cost, or increase the efficiency, of present management practices.