California Wetlands Information System

Vernal Pools:

Their History and Status in California's Central Valley

What is a Vernal Pool?

Vernal pools are seasonally flooded depressions found on ancient soils with an impermeable layer such as a hardpan, claypan, or volcanic basalt. The impermeable layer allows the pools to retain water much longer then the surrounding uplands; nonetheless, the pools are shallow enough to dry up each season. Vernal pools often fill and empty several times during the rainy season. Only plants and animals that are adapted to this cycle of wetting and drying can survive in vernal pools over time.

These specialized plants and animals are what make vernal pools unique. As winter rains fill the pools, freshwater invertebrates, crustaceans, and amphibians emerge. Vernal pool plants sprout underwater, some using special floating leaves and air-filled stems to stay afloat. Some of these plants even flower underwater! Birds arrive to feed on the vernal pool plants and animals.

In spring, flowering plants produce the brightly-colored concentric rings of flowers that vernal pools are famous for. Native bees nest in vernal pools and pollinate pool flowers. Insects and crustaceans produce cysts and eggs, and plants produce seeds that are buried in the muddy pool bottom. The mud protects cysts, eggs, and seeds from the hot, dry Central Valley summer. By late summer, amphibians have dug deep into the soils and gone dormant, awaiting the next rainy season. Vernal pools have completely dried out and most of the plant and animal species have either disappeared into the soils or set seed and died. In this phase, vernal pools are really "banks" full of resting seeds, cysts, and eggs that can survive through summer, and even extended droughts, until the onset of the rains begin the life cycle anew.

Why Do Some People Think Vernal Pools Are Important?

Vernal pools are considered an important natural resources in California and worldwide.

Where Are Vernal Pools Found?

California's vernal pools occur on a variety of landscape formations, most often on alluvial formations deposited by ancient waterways and seas. The greatest extent of this type of landscape formation is in our Central Valley, in areas where alluvial surfaces were exposed after the retreat of the inland sea during the Pleistocene era. Similar alluvial landscape formations occur in inland valleys of the inner Coast Ranges, and along coastal terraces of Southern California, where geologic forces have lifted the original alluvial landscape surfaces above sea level.

A second type of landscape formation with vernal pools are ancient volcanic mudflows, where rapid weathering of volcanic materials have formed dense clay soils and bedrock restricting layers near the soil surface. Volcanic landscape formations are found in northeast California and in the northern end of the Sacramento Valley.

Why Are Vernal Pools Regulated and Who Regulates Them?

Federal, state, and local laws and policies regulate certain activities in wetlands, including vernal pools, and may prohibit activities that could harm or harass threatened or endangered wildlife species or migratory waterfowl. In addition, the California Environmental Quality Act, a state law, requires public agencies to consider the effects of proposed actions on biological resources. Environmental laws and policies that pertain to vernal pools in California include:

These environmental laws and policies require agencies and landowners to consider the effects of a proposed action on the environment; determine if a less damaging alternative is available; obtain and comply with necessary wetland permits; and comply with endangered species laws (for vernal pools that support endangered species). Compensatory mitigation may be required by local Planning Departments or regulatory permitting agencies to help ensure that the project does not result in a loss of wetland functions.

This brochure was prepared by the

San Joaquin County

Resource Conservation District

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