Temporary pond formed by the collection of
rain water in areas with poor
drainage (often a
clay soil). The standing water only lasts through
spring (thus,
vernal), as warmer weather
evaporates the water. The alkaline soils create
ecological islands, often with plants uniquely suited for the brief growing season.
In California, a distinguishing feature of the pools is the presence of wildflowers, many of them endemic and many endangered.
Different states have differing legal definitions of vernal pools, based on size, minimum length of time that water must be present, and the indicator species that are allowed to "certify" a seasonal wetland as a vernal pool; all of this is in an effort to regulate and protect these seasonal microhabitats.