Vernal Pools

San Diego supports a unique set of ecosystems referred to as vernal pools.  Soil depressions on the mesas regularly fill with rainwater and then dry out during the summer months.  Starting with the first rains, a variety of uniquely adapted plants and animals emerge from the soil to create a wetland community that lasts only a few months before going dormant, including the endangered San Diego Mesa Mint, San Diego Button Celery, Otay Mesa Mint, San Diego fairy shrimp, and Riverside fairy shrimp.

San Diego vernal pool                                                  Southern California Chaparral Field Institute

Vernal pools are usually quite small, ranging in size from small puddles to shallow lakes and are usually found in a gently sloping plain of grassland. Although generally isolated, they are sometimes connected to each other by small drainages known as vernal swales. Beneath vernal pools lies either bedrock or a hard clay layer in the soil that helps keep water in the pool.


San Diego Mesa Mint                                              San Diego Button Celery                                                    Otay Mesa Mint

2001 Ellen Friedman & Ted Dunning                                    Copyright © 2001 Greg Mason                                          Copyright © 2001 Greg Mason

Climatic changes associated with each season cause dramatic changes in the appearance of vernal pools. The pools collect water during winter and spring rains, changing in volume in response to varying weather patterns. During a single season, pools may fill and dry several times. In years of drought, some pools may not fill at all. In the spring, wildflowers often bloom in brilliant circles of color that follow the receding shoreline of the pools. By early summer, the water has evaporated, and the clay pools appear brown, barren, and cracked.

The unique environment of vernal pools provides habitat for numerous rare plants and animals that are able to survive and thrive in these harsh conditions. Many of these plants and animals spend the dry season as seeds, eggs, or cysts, and then grow and reproduce when the ponds are again filled with water. In addition, birds such as egrets, ducks, and hawks use vernal pools as a seasonal source of food and water.

Vernal pools are a valuable and increasingly threatened ecosystem. More than 90% of California's vernal pools have already been lost. Great efforts are being made to protect the remaining vernal pools, as their disappearance marks the loss of rare and important habitat and some of the associated plant and animal species as well.

Vernal Pool         Photo by U.S. Evironmental Protection Agency


California Vernal Pool Regions

(California Wetlands Information System*)


Betzler, J., Diffendorfer, J., Fleury, S., Hawke, M.A., Klein, M., Morrison, S., Nichols, G., Oberbauer, T., Rochester, C., Webb, M., Williams, K. (2003, November 14). A summary of affected flora and fauna in the San Diego County fires of 2003 (BAER Report). Retrieved December 30, 2004, from

California Native Plant Society. (2005). Wetland and vernal pool issues (CNPS Internet Links). Retrieved February 20, 2005 from

*California Wetlands Information System, California Resources Agency. (2005). California vernal pool regions (map). Retrieved February 20, 2005 from

University of California, Berkeley. (2005). CalPhotos (Berkelely Digital Library Project). Retrieved February 20, 2005 from

U.S. Environmental Protection Agency (2005). Wetlands; vernal pools. Retrieved February 20, 2005 from


Photo Credits:

Friedman, E., & Dunning, T. (2001). San Diego mesa mint. Retrieved February 20, 2005 from CalPhotos: Plants

Mason, G. (2001). Otay Mesa mint. Retrieved February 20, 2005 from CalPhotos: Plants

Mason, G. (2001). San Diego button celery. Retrieved February 20, 2005 from CalPhotos: Plants

Southern California Chaparral Field Institute. (2005). Photo of vernal pool. Retrieved April 24, 2005 from

U.S. Environmental Protection Agency (2005). Retrieved February 20, 2005 from