Vernal Pools

Vernal Pools are one of the more fascinating features of the California landscape. A vernal pool is a temporary body of water that is dry part of the year. In California they are formed during the winter rains and then go dry over the summer.

Navarretia leucocephala
ssp. bakeri

These pools are home to very specialized plant and animal communities, uniquely adapted to the challenging and rapidly changing conditions of the pool. They are especially known for their rare and diverse populations of amphibians and aquatic invertebrates, made possible by the lack of fish. They also harbor really interesting annual flowering plants that put on an ever-changing show as the pool dries up every summer.

Plagiobothrys bracteatus, vernal
pool popcorn flower

The most famous vernal pools are found in California's Central Valley, where very few remain after over a century of development, but we have one relatively close to San Francisco, at Mount Burdell Open Space Preserve, in Marin County: Hidden Lake.

Navarretia leucocephala
ssp. bakeri and
Juncus xiphioides

I visited Hidden Lake in late May just as the first ring of wildflowers was blooming along the eastern edge of the receding lake: Navarretia leucocephala ssp. bakeri. There were drifts of this tiny white flower starting to bloom, turning the edge of the lake white. Interspersed among the Navarretia was Plagiobothrys bracteatus, vernal pool popcorn flower, along with Juncus xiphioides, Iris-leaved Rush. Another amazing occurrence: tens of thousands of Pacific Tree Frogs swarming all around the edge of the lake - a fantastic sight.