Thompson Reservoir (Lake)

Reachcode: 17120005001399 | Area: 2159.2 acres | Shoreline: 26.7 mi | View on Interactive Map

(From Atlas of Oregon Lakes, Johnson et al. 1985) Thompson Valley Reservoir is a large irrigation impoundment located on the upper East Fork of Silver Creek in the Fremont National Forest, approximately eight miles downstream of the Silver Lake Diversion Dam. The dam and reservoir were constructed about 1930 for the Silver Lake Irrigation District. It is a shallow reservoir which, at full pool, covers an area of about 1800 acres and contains 7750 acre feet of water. Water is withdrawn for downstream irrigation through the summer and the surface area decreases considerably. In some years, the reservoir is nearly dry by September. Surface inflow to the reservoir is from a number of intermittent streams. The contributing drainage basin is covered in large part by a pine forest, and much of the area is used for grazing. Although the reservoir is within the National Forest, about half the shoreline is privately owned land. The reservoir is stocked each spring with about 200,000 fingerling rainbow trout, and the growth rate is reportedly extremely rapid. The only other major fish species is the tui chub or roach (Gila bicolor), and it greatly outnumbers the trout. Fishing pressure is normally high, but the high population of rough fish has resulted in a decline of trout. Although treated periodically with rotenone, the roach quickly repopulate to the detriment of the trout. Road access to the reservoir is very good, and there are two good Forest Service campgrounds. However, because of the fish problems the facilities are not heavily used. The water quality in Thompson Valley Reservoir is good, and it is classified as mesotrophic. The total phosphorus concentration is higher than might be expected for a mesotrophic reservoir, but a low chlorophyl concentration and high water transparency indicate low densities of phytoplankton. McHugh (1972) reported low densities of Asterionella formosa in the spring and low densities of Ceratium hirundinella and Melosira granulata in the summer. Macrophytes are scarce, due to extreme water level fluctuations.