vernal pool

A shallow, seasonal wetland that fills with winter rain on top of an impermeable hardpan layer, holds water through spring, and dries completely by summer — leaving a cracked, bare depression that gives no sign of what it held. In the weeks between filling and drying, vernal pools support an extraordinary community of life found almost nowhere else: fairy shrimp, tiger salamanders, specialized wildflowers that bloom in concentric rings as the water recedes. Most of California's Central Valley vernal pools have been destroyed by development and agriculture. The ones that remain are among the most endangered ecosystems on the continent.