Twain Harte Lake is currently being drained after part of the dam's mooring appeared to have cracked this morning, prompting warnings from the National Weather Service about the potential for flooding downstream.
State engineers were en route this afternoon to inspect the dam to determine the type and extent of the damage. Initial inspections by the dam's operator - the Twain Harte Lake Association - found that part of the rock footing holding one side of the dam had broken, allowing water to leak through.
To prevent further damage and make examination easier, the lake level is being lowered 10 to 15 feet, which would put the lake's surface below the cracked area. It could take two days to drop the lake level that much.
The slow release is intended to prevent flash flooding downstream along Sullivan Creek. Phoenix Lake is being lowered to make room for the water.
Twain Harte Lake, a popular summer swimming hole, is currently closed to the public.
The dam breakage was discovered this morning by an association employee, who observed water pouring through a crack in the dam, followed by a loud bang.
About 10:45 a.m., the National Weather Service issued a flash flood warning for areas immediately adjacent to Sullivan Creek downstream from Twain Harte Lake. It was effective through 10:45 p.m.
At this point, there is no danger of the dam breaking, said Twain Harte Lake Board President Betty Jesperson.
Jesperson added that the dam just a few weeks ago had cleared a routine safety inspection by the state Department of Water Resources Division of Dam Safety.
The metal-and-concrete dam was built in 1928 and holds back water from a 1-suqare-mile patch of the Sullivan Creek watershed. From Twain Harte Lake, the water travels to Crystal Falls Lake and then Phoenix Lake.
Twain Harte Lake covers a 12-acre area but has a storage capacity of 143 acre-feet, or enough water to cover 143 acres to a depth of one foot.
Tuolumne Utilities District, Cal Fire, Twain Harte Fire and county Office of Emergency Services are on scene.