A sag pond is formed along a strike-slip fault, which may create a depression in the earth. When water enters the depression from rivers, streams, rainfall or snowfall, it fills the low-lying area, and a pond is the result.
Sag pond formation is common in California, and many of them can be found on the San Andreas Fault, which runs through the western part of the state. Aerial photography or high-resolution topography can easily determine their locations.
Sag ponds have been converted into stock reservoirs. Larger sag ponds are converted into reservoirs for public water sources, such as San Andreas Lake
Sag ponds of various sizes can be found along the San Andreas Fault, most notably within the Carrizo Plain and Sierra Pelona Mountains. Some have been converted into reservoirs like the San Andreas Reservoir near San Bruno, California.