- Douglas County)
(From Atlas of Oregon Lakes, Johnson et al. 1985). Diamond Lake, a large and heavily used lake in the southern Oregon Cascades, lies at an elevation of 5183 feet in a scenic glacial valley between Mt. Thielsen and Mt. Bailey. The lake was not named for its shape as is commonly believed, but for John Diamond, the same man for whom Diamond Peak was named. He discovered the lake from the summit of Diamond Peak in 1852. Pleistocene glaciers scoured out the Diamond Lake basin, creating a broad valley which was blocked about one million years ago by lava flowing from the slopes of Mt. Thielsen. As the glaciers retreated some 10,000 years ago, meltwater filled the shallow valley behind the lava barrier, thereby creating Diamond Lake. Its morphology is similar to that of other ice-scoured lakes in Oregon; that is, an elliptical form with one end shallower than the other. The lake bottom slopes gradually and uniformly to the deepest point (53 feet) north of center. Other lakes in this survey which exhibit similar morphological characteristics include Suttle, Cultus, Odell, and Crescent Lakes in the Cascades, and Wallowa Lake in northeast Oregon.
Diamond Lake and its drainage basin are entirely within the Umpqua National Forest. The topography and geology of the area is dominated by its volcanic history. The drainage basin is flanked by the High Cascade volcanoes, Mt. Thielsen (9183 feet) and Mt. Bailey (8363 feet). Tiber Crater (7402 feet) marks the southern point of the drainage basin. Soils are porous and well-drained and include glacial debris as well as pumice and ash from the eruption of Mt. Mazama. The bedrock is basaltic, the remnant of lava flows from Mts. Bailey and Thielsen. A coniferous forest covers the landscape and is composed primarily of lodgepole pine, mountain hemlock, and species of fir. Understory vegetation includes huckleberry, grasses, sedges, and manzanita. At the south end of the lake is a large marsh grading into shallow water area.
The major inlet streams to Diamond Lake are Silent Creek from the southwest and Short Creek from the southeast. Other tributaries are primarily intermittent and contribute significant flow only from snowmelt. Surface outflow is continuous into Lake Creek, a tributary of the North Umpqua River, and the rate of outflow is regulated by the Oregon Department of Fish and Wildlife. Flashboards are generally installed in Lake Creek after spring runoff to maintain the lake level during summer, and then are removed in fall. Although the lake is relatively shallow for its size, the hydrologic retention time is quite long, averaging 1.6 years, because of the comparatively small size of the contributing drainage basin.
Diamond Lake receives a great amount of recreational use for swimming, fishing, boating, and camping. Winter activities, such as snowmobiling on ice, are also very common. On the shoreline are two private lodges, over 100 seasonal dwellings, three Forest Service campgrounds, and two picnic areas to accommodate visitors. A Forest Service road encircles the lake, providing access at all points. Recreation has increased steadily during the past several decades; in 1923, the lake had 30,000 visitor-days; this increased to 104,000 between 1956-1963, and to 307,000 visitor-days in 1965, and finally in 1977 the lake received about 670,000 visitor-days (Lauer et al 1979). Diamond Lake has a well-established reputation for its large rainbow trout, although this was not always the case. For several years prior to 1954 the fishery had been declining; in that year the lake was treated for an abundance of trash fish. Following treatment it was stocked with Kamloops rainbows, a Canadian strain that provided spectacular fishing for several years. However, the Kamloops fishery eventually proved to be unsatisfactory because the fish did not attain the desired size and were difficult to catch. About 1970 the Oregon Department of Fish and Wildlife began stocking high grade strains of local rainbow trout. Since then excellent catches have been made with all methods of angling.
Diamond Lake develops thermal stratification during the summer, although surface temperatures remain cool and the onset of stratification comes rather late in the season because of the high elevation. The lake is ice-covered in the winter and develops a reverse stratification under the ice. Oxygen is depleted below 30 feet depth (10 meters) during both summer and winter stratification. The concentrations of major ions in the lake are somewhat above average for alpine lakes, but typical for Cascade Mountain lakes surrounded by pumice soils. Conductivity (29 umhos/cm) and alkalinity (15 mg/1) are also moderate. During summer, surface pH sometimes exceeds 9 because of the growth of planktonic algae. The concentration of phosphorus in the lake varies substantially from year to year and season to season, but on the average is high enough to cause the lake to be moderately eutrophic.
During the 1920s a sewage collection system was constructed around Diamond Lake to prevent nutrients from human wastes from reaching the water. In 1979 the Environmental Protection Agency completed a thorough study of the impact of this waste-water interception program. The purpose of the study was to define the nutrient budget and describe the benthic macroinvertebrate population (Lauer et al 1979). Conclusions were as follows:
1. Diamond Lake is mesotrophic to eutrophic as a result of natural nutrient loading from the tributaries, groundwater and bottom sediments. Silent Creek supplies over half the surface runoff to the lake, and Short Creek supplies about 20 percent of the total. The average concentration of phosphorus in these two streams is more than 0.050 mg/1, or enough to support eutrophic conditions in the lake. Inflow of groundwater is thought to contribute approximately 10 percent of the phosphorus entering the lake. In addition, the sediment in the lake is rich in nutrients; these are mobilized by the rooted macrophytes that cover as much as 50 percent of the bottom.
2. The nutrient contribution from cultural activites in the drainage basin is, at present, relatively insignificant compared to natural sources. Heavy use for water-based recreation undoubtedly contributes additional nutrients to the lake. In particular, the common use of canned corn, processed cheese, canned cat food, and other food items such as fish bait or "chum" contributes additional nutrients.
3. During the period of investigation no change was found in the trophic status of the lake that could be attributed to the wastewater diversion system.
4. The trophic status of Diamond Lake is appropriate in satisfying present recreational demands..Because of the natural loading of phosphorus, algal blooms frequently develop in the lake. Species commonly observed include eutrophic diatoms and a nitrogen fixing bluegreen alga (Anabaena circinalis). The phytoplankton sample of 8/22/82 had a total density lower than in previous studies on Diamond Lake, but the species present were .similar. It may be that this sample is atypical of the average phytoplankton densities. The high growth rates of planktonic algae support a diverse and productive food chain, thus explaining the highly successful development of the trout fishery. In sum, all trophic indicators suggest an eutrophic classification for Diamond Lake.