Vernal pools occur on coastal terraces in southern California, alluvial deposits in the Central Valley and eroded lava flows throughout the state and are biologically unique in each area. They are seasonal wetlands that occur in depressions where there is an underlying impermeable layer that becomes saturated during the winter rains and ponds through the spring. Vernal pools dry up by early to late spring, depending on the season's rainfall timing and amount. As the water recedes, rings of vernal pool adapted species become apparent. Pools stay desiccated through the summer and early fall and associated species go dormant. A unique suite of both plant and animal species are adapted to this cyclical wetting and drying, including coyote thistle, dwarf woolyheads, Fairy Shrimp, Clam Shrimp and Tiger Salamanders. Vernal pools also provide important resting and feeding places for migratory waterfowl. Vernal pool habitat is becoming increasingly rare: more than 90% of vernal pools in California have been destroyed due to agriculture, over-grazing, introduced weeds, and urban development.