Introduction to Vernal Pools

What are Vernal Pools?

California vernal pools are a rare type of wetland that exists in very few places on Earth. Around Sacramento, the pools are found in rolling grassland. What makes vernal pools different from other wetlands, ponds or lakes is that they are temporary pools. They are totally dry for eight months of the year during our dry season. In fact in order to have vernal pools, you need a wet season and a dry season, just like the Mediterranean climate of California.

In Sacramento most rain falls in the winter between December and March. This is when the vernal pools fill with water. Once the rain stops in the spring, the pools begin to evaporate. By the end of April, the pools fill with tiny flowers growing from the once muddy bottom. Vernal pools are so colorful in spring, they are named for spring: Vernal means “spring” in Latin.

Within a few weeks all of the flowers have made their seeds and the plants have dried up. By July all that remains of the vernal pools is dried, cracked soil and a carpet of short, brown plants. The pools rest like this for another six months until the winter rains return. Then the cycle begins again.

Wet Phase
Flower Phase
Dry Phase

The Vernal Pool Grassland

When rain falls on a vernal pool grassland, some water sinks into the ground and the rest flows slowly over the land as runoff. This runoff flows to streams or into depressions (low places) in the grassland. The water cannot move deeper into the ground in a vernal pool grassland because hardpan blocks its path.

Hardpan is a layer of clay or minerals that water cannot pass through easily. The hardpan can be a few inches to a few feet below the ground surface. Under the grassland, the hardpan acts like the bottom of a bathtub holding up the water. As winter rains continue, rain and runoff saturate (fill with water) the soil above the hardpan. The water perches (sits) on the hardpan. In the upland (the higher, drier areas of vernal pool grasslands) we cannot see this water because the water table is below the soil surface. The only place we can see the perched water is in the depressions we call vernal pools.

The only way for vernal pools to empty is by very slow movement of water through the ground or by evaporation. This can take days, weeks or months depending on the amount of rainfall, the air temperature and the size of the pool. While some vernal pools are bigger than a playground, many are no larger than a classroom. No two pools are exactly alike.

Throughout the uplands are scattered large humps of soil called mima mounds. Nobody knows exactly how mima mounds and vernal pools formed because it happened long ago. It is likely that earthquakes, volcanoes and floods all helped shape the present land surface over the last half a million (500,000) years. Since human beings did not arrive in California until about 12,000 years ago, vernal pools have been a part of our landscape far longer than people.

View of mima mounds and vernal pools from a small airplane.

Vernal Pools and Human History

For many thousands of years tribes of native people came to the vernal pool grasslands to collect food. As recently as 1868, conservationist John Muir described his first view of spring in the Central Valley vernal pool grassland. —Sauntering in any direction my feet would brush about a hundred flowers with every step, as if I were wading in liquid gold.— He carefully noted that this natural flower garden was nearly 400 miles long and 30 miles wide. 


Within 125 years of his visit, up to 90 percent of California’s vernal pools were gone. Most had been drained and plowed to feed the ever-growing population of California and the nation. In the furrows left by the plow, farmers still find the stone mortars and pestles of the native people who had used the land before them.

mortar.jpgMost of the vernal pools we find today occur in the few large cattle ranches that remain in California. As these ranches are converted to vineyards and new communities, more vernal pools disappear. Military bases are the other refuge for vernal pool grasslands. Much of the land within their fences was not developed during the 1900s. As these bases are converted to non-military uses, roads and buildings threaten these vernal pools too.

Looking into a vernal pool is like looking back in time. These temporary wetlands look much like they did over 100,000 years ago. Visiting a vernal pool is like walking into a time when animals roamed this land and there were no people. You can see a piece of what John Muir described and explore a unique part of California’s natural heritage. When you are done, perhaps you can answer a question many people ask, “What good are vernal pools, anyway?”