Vernal Pools

Yesterday we went on a field trip to visit some vernal pools not far from the house. John's seven-year-old niece, Kayley, was my companion for the day.

Vernal pools are seasonally flooded depressions found on ancient soils with an impermeable layer such as hardpan, claypan, or volcanic basalt. The impermeable layer allows the pools to retain water much longer then the surrounding uplands, but they are shallow enough to completely dry up during the summer and fall. Only plants and animals that are adapted to this cycle of wetting and drying can survive in vernal pools over time.

The above photo is from the EPA Web site so you can see what the vernal pools like like when they are still in the wet phase, going into the flowering phase. The specialized plants and animals found in vernal pools are what make the pools unique. As winter rains fill the pools, freshwater invertebrates, crustaceans, and amphibians emerge. Many of these animals live no where else on the Earth and are important food sources for migratory waterfowl. Vernal pool plants sprout underwater, some using special floating leaves and air-filled stems to stay afloat. Some of the plants even flower underwater.









We saw the pools during the flowering phase which occurs each Spring. Flowering plants produce the brilliant circles of color that follow the receding shoreline of the pools. Flowering vernal pools are like snowflakes in that no two are exactly alike. There are over 60 different plant species that live in California vernal pools. The peak bloom of each plant species coincides with the emergence of the pollinator insect that will visit the plant.


Since this year has been drier than recent years, the water in the pools have already evaporated, and in some places the ground is brown, barren, and cracked, indicating that they are quickly going into the dry phase. Even when they appear dry and barren, the grasslands and vernal pools support many animals throughout the summer and fall. Seeds left behind from the flowering plants provide food for insects, birds, and gophers.

We walked, and walked through the grasslands and visited as many of the pools as we could. The small wild flowers are protected by Federal and State laws, so since we couldn't pick them, Kayley decided to pick rocks instead. By the end of our field trip, both of her pockets were full with rocks. I don't think Kayley had ever walked that much in her entire life, but she must have enjoyed herself because she said that she wanted to come back in early spring next year so she could see the water and animals in the pools.

Although it was a little windy, it was a beautiful day, and we could see the snow-capped Sierras off in the distance.