Vernal pool

Seasonally-flooded depressions found on soils with an impermeable layer such as a hardpan, claypan, or volcanic basalt. California's vernal pools occur on a variety of landscape formations, but most often on alluvial formations deposited by ancient waterways and seas. The impermeable layer allows the pools to retain water much longer than the surrounding uplands; nonetheless, the pools are shallow enough to dry up each season.