Clear Lake Reservoir is a reservoir in northwestern Modoc County, California in the Modoc National Forest about 40 mi (64 km) northwest of Alturas. It is formed by Clear Lake Dam on the Lost River and has a capacity of 527,000 acre feet (650,000,000 m3). The reservoir is not be confused with Clear Lake, a large natural lake on Cache Creek in Lake County, California.
The original dam was constructed of rockfill and was completed in 1910. It was 42 feet (13 m) tall from the bottom of the foundation to the crest, 32 ft (10 m) if measured from the crest to the original stream bed. It was replaced with a concrete dam near the old dam in 2002. The old dam had deteriorated and the water level was kept low to prevent it from failing. Its crest is at the same height as the old dam and the reservoir's capacity is about the same. There is a small hydroelectric plant at the dam with a capacity of 3 MW. The dam is owned by the United States Bureau of Reclamation. It was built to reduce flows into the reclaimed wetlands of Tule Lake. It also reduces flows into Tule Lake Sumps in the Tule Lake National Wildlife Refuge. The reservoir created by the dam has a very large surface area, 25,760 acres (104.2 km²), and its average depth at maximum capacity is only about 20 feet (6 m), so it has a very high rate of evaporation. In 1998, 74,500 acre feet (91,900,000 m3) of water from the reservoir evaporated, more than any other lake in the state besides Shasta Lake, which has over 8 times the volume. Because of this, Clear Lake Reservoir is not an efficient water storage reservoir. This is a problem since the dam now provides water for irrigation of the eastern half of the Klamath Basin. The reservoir is in Modoc National Forest and Clear Lake National Wildlife Refuge, so recreation opportunities are limited.