|National Weather Service|
A flash flood warning in the Sierra Nevada town of Twain Harte has been cancelled this evening after earlier concerns over a possible dam failure. Twain Harte Reservoir is a small (143 acre foot) dam that drains into Sullivan Creek and Don Pedro Dam. People reported hearing loud "booming" noises and seeing new cracks in the bedrock around the damsite. Water was said to be leaking from the dam. Authorities closed the lake and are monitoring the situation, but are now saying that dam failure is unlikely.
My impression of pictures posted on facebook and in early news reports is that the fractures in the granite are exfoliation sheets, and I can't tell if they are new or old. Exfoliation or jointing occurs as the once deeply buried rock is exposed at the surface and the pressure of burial is released. The rock fractures, often into slabs that run parallel to the surface. Such slabs are often the source of rockfalls along cliffs like those of Yosemite Valley.