Castle Lake is a glacial lake (cirque lake or tarn) located in northern California along the eastern edge of the Klamath Mountains, in Siskiyou County, near the city of Mount Shasta. The outlet of the lake drains into Castle Lake Creek, and then into Lake Siskiyou; the lake is part of the headwaters of the Sacramento River. Nearly all of the lake's 47 acres (19 ha) are within the Shasta-Trinity National Forest. Trails lead from Castle Lake into the adjacent Castle Crags Wilderness area, and on to Castle Crags State Park, including trails to Little Castle Lake and Heart Lake. Fishing, camping, and hiking are also available at or near the lake. The lake and the surrounding area contain a wide variety of animals and plants including trout, bears, deer, otters, frogs, and osprey. The area is also well-known for early summer displays of flowers, including red Columbine (Aquilegia truncata), fawn lily (Erythronium sp.), and Shasta pentstemon (Pentstemon laetus).