MORE ABOUT VERNAL POOLS

A million years of geology and evolution have produced the spectacular vernal pools at Sacramento's Mather Field. The process that formed these 50,000 to 100,000 year old pools began over a million years ago when volcanic eruptions spewed ash over an ancient river bed. Minerals in that ash eventually formed a layer of hardpan under the soil surface. This layer prevents rainwater from draining out of shallow depressions in the grasslands. The resulting temporary pools remain wet for several weeks to a few months, as their waters slowly evaporate.

A Threatened Home for Special Species

California's unique vernal pools are one of the rarest and most threatened ecosystems in the world. Vernal pools in our Mediterranean climate host a unique assemblage of plant and animal species adapted to life in this environment of extremes. Each must tolerate months of winter flooding followed by the desert-like heat and drought typical of Sacramento's summers. Most of these species are native to California and survive only in vernal pools. As their vernal pool habitat continues to be destroyed, they too will disappear.

Vernal Pool Biology

In winter and early spring, the pools provide an oasis for scores of water-dependent wildlife. Frogs, toads, salamanders, crustaceans, aquatic insects and smaller creatures race against the clock to complete their short life cycles prior to the pools drying up. Teaming with life, the pools provide essential food for many birds and mammals.

Vernal Pool Plants

Specialized vernal pool plants are adapted to begin their growth underwater. Some 200 plant species grow in vernal pools. Half of these are rarely found outside this unique habitat. However, any given pool typically supports only 15 to 20 species, in an unpredictable array of combinations. In that way vernal pools are a lot like snowflakes; botanically speaking, no two are alike.

As the pools dry down, the newly exposed plant seedlings grow quickly, springing into bloom from March to May. The flowers form colorful concentric rings and dense patches of yellow, white, pink, and purple flowers. The display changes from week to week, as each species gives way to the next, like a floral kaleidoscope. These spring or "vernal" flower displays give vernal pools their name.

During the hot, dry summer months, vernal pools lie brown and barren. Often they are the only open patches in the grasslands, devoid of the thatch of non-native grasses that crowds the upland areas. The eggs, cysts and seeds of hundreds of vernal pool-dependent species rest in the sun-baked mud of the pool bottoms, awaiting the next winter rains. Young solitary bees develop in their pool-side underground chambers, awaiting the bloom of the one flower species they require to feed the next generation.

Vernal Pool Geology