- Klamath County)
(From Atlas of Oregon Lakes, Johnson et. al 1985) Agency Lake, so named because of the Klamath Indian Agency nearby, is actually the north arm of Upper Klamath Lake and together they constitute a single hydrologic entity. Agency Lake receives surface runoff from a relatively small area (about 244 square miles), only a fraction of the total 3,800 square miles that drain into Upper Klamath Lake. The water surface elevation of the entire water body is controlled at the outlet from Upper Klamath Lake so that it varies from about 4136 feet to 4143 feet above sea level. Fluctuations are due to withdrawals for irrigation and downstream power production. Wood River is the primary inflow into Agency Lake, with an average annual runoff of 171,800 acre feet as measured at Fort Klamath. It is a spring-fed river and thus maintains a fairly constant flow throughout the year.
The majority of the Agency Lake drainage basin is in private ownership; portions to the east of Wood River are in the Winema National Forest and the upper reaches in Crater Lake National Park. The Klamath Lake National Wildlife Refuge adjoins the lake on the southwest and the entire lake environs are a good waterfowl habitat. About 31 percent of the drainage basin is agricultural land irrigated with water from Wood River, Fourmile Creek and Sevenmile Creek.
Agency Lake is extremely shallow for its size and is frequently exposed to strong winds. The water is constantly well mixed and does not develop any persistent temperature stratification. Frequent mixing also leads to suspension of bottom sediments. The concentrations of ions in the lake are slightly above average for freshwater lakes in the state and the pH of the water is frequently above 9 during the summer when heavy algal blooms develop. Primary productivity in the lake is high and it is a good producer of rainbow and brown trout which pass through to Wood River and Sevenmile Creek. This shallow lake is hypereutrophic because of the high concentrations of nutrients from Wood River, and from nutrient rich sediments. The hypereutrophic condition is a result of natural processes and no significant modification appears possible at any realistic cost. The frequent blooms of blue-green algae, primarily Aphanizomenon, are stimulated by the heavy loading of phosphorus to the lake. Although the concentrations of nitrate and ammonia are not high, Aphanizomenon is a nitrogen fixer (that is, it changes atmospheric nitrogen to nitrate and ammonia) and by this process supplies this essential nutrient to the lake. Aphanizomenon appears much earlier in the season and reaches higher densities than in Upper Klamath Lake. Agency Lake thus has a higher trophic state, indeed one of the highest in Oregon.