Vernal Pools

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.

Fascinating Features

  • 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.

Habitat Values

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

Although most vernal pool critters are very small, they are an important food source for shore birds and waterfowl. Other birds, like the Great Blue Heron (Ardea herodias), feed on amphibians that are attracted to the pools for breeding; geese eat vegetation that grows around the pools; and birds, like Cliff Swallows (Petrochelidon pyrrhonota), use the mud found around the edges of drying pools to build their nests.

Amphibians

Amphibians such as the western spadefoot (Spea hammondii), California tiger salamander (Ambystoma californiense), and Sierran treefrog (Pseudacris sierra) make use of the vernal pool’s brief period of inundation for breeding and laying eggs. As the pools begin to dry down, tadpoles and salamander larvae begin to metamorphose, sprouting legs that will carry them away from the pools into soil cracks or burrows.

Pollinators

Vernal pool plants burst into flower, attracting specialized bees and other insects, some of which collect pollen from only a single type of flower, to carry out the important business of pollination.

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.

Mather Field Vernal Pools

Three publicly accessible walking tours are associated with the Illa M. Collin Conservation Preserve at Mather Field. Walking Tour #2 will take you to an area of vernal pool complex.

Jepson Prairie Preserve

A short, self-guided trail winds through upland California prairie and to the edge of Olcott Lake, a large vernal pool. Docent-led tours are available in the spring through the Solano Land Trust.