In spring, colorful fairy rings of diminutive wildflowers encircle large and small pools that are beginning to dry down after the fall and winter rains. Vernal pools are temporary wetlands that hold water for several months, due to an impermeable hardpan or clay layer, and eventually dry out as the weather warms. These specific conditions resulted from millions of years of soil formation as minerals percolated down through the upper soil layers, creating the hardpan. Vernal pools occur only where a Mediterranean climate of hot, dry summers and cool, wet winters results in a predictable wetting and drying cycle.
- The name vernal pool is derived from the Latin word vernus, meaning spring, perhaps because spring is the season in which vernal pools are most noticeable.
- Vernal pools can be as small as 30 square feet to very large, like Olcott Lake in Solano County, which can cover more than 100 acres in wet years.
- Outside of California, vernal pools that function similarly to those within the state occur in only a few other places—southern Oregon; Baja California, Mexico; and the Cape region of South Africa. Vernal pools in these areas exhibit distinct flora and fauna unique to their regions.
Vernal pools first spring to life as rainwater drains across the prairie landscape to fill shallow depressions. Seeds of vernal pool plants begin to germinate under these conditions, which requires them to grow as submerged plants, with special adaptations such as floating leaves and hollow stems that allow oxygen to be transported to the roots. Vernal pool aquatic invertebrates hatch at this time, too, after lying dormant as cysts in the dry summer soils.
Important food source
Places to Visit
There are several locations around the Sacramento Valley that are open to the public, where you can see and learn about vernal pool habitat for yourself. Here are a few of our suggestions.