Vernal pools of the Santa Rosa Plateau

Date: 2/16/01
Pool Depth: 2 inches

Date: 3/1/01
Pool Depth: 16 inches

Date: 6/4/01
Pool Depth: 0 inches

Date: 10/2/01
Pool Depth: 0 inches

See Pictures Vs. Time for 2001, 2002 (a year without a pool), and 2003.

Although much of the information on this page applies to all the vernal pools at the Santa Rosa Plateau Ecological Reserve, this page primarily discusses the Main Pool on the Mesa de Colorado. This is the largest and best vernal pool at the Santa Rosa Plateau Ecological Reserve, and, fortunately, happens to be the only one that can be visited by the public, traversed in part by a boardwalk on the wheelchair-accessible Vernal pool Trail. It is also the only pool I have studied in detail, with over a decade of detailed observations.

Vernal is derived from the Latin word for spring. A vernal pool is seasonal pool that is present in the spring and dries up in late spring or summer. It doesn't have water in it again until winter rains recreate the pool. This makes it fundamentally different from a year-round lake. In particular, fish are absent, since they require year-round water.

The pools at the Santa Rosa Plateau usually, but not always, fill with water each winter (see historical records), with the water level declining at the relatively-slow rate of one inch per week in the absence of further rain. This feature makes them much more interesting than puddles that fill with water and then go dry! It allows a community of plants and animals to be adapted to that kind of pool, and thus to put on a regular show each year. Only specially-adapted plants and animals can survive the long period of inundation, and the longer period of dryness and heat.

Vernal pools usually are found in areas where the winter is the rainy season, and summer is the dry season. The pools fill in winter from the rains, and vanish in the dry heat of summer from a combination of percolation and evaporation. Two other factors are required to form a vernal pool that can support these specially-adapted plants and animals: a depression that can collect water, and something to restrict drainage so that the water can remain on the surface for a period of weeks to months.

There are many types of vernal pools in California. The California Wetlands Information System has put together a map showing their distribution. A detailed map of the individual pools in each region along with very detailed descriptions of the type of vernal pools, their protection status, geology, etc. used to be available on their site, but apparently is no longer online.

The vernal pools of the Santa Rosa Plateau are the only known examples of Southern Basalt Flow Vernal pools. The basalt on the Santa Rosa Plateau is the key to the large number of vernal pools there, since intact basalt is nearly impervious to penetration by water. Only 14 vernal pools survive in Riverside County; 13 of them are protected on the Santa Rosa Plateau. Of the 13 pools at the Santa Rosa Plateau, eight are on Mesa de Burro, four are on Mesa de Colorado and one is on Mesa de la Punta.

The basalt came from a spreading center that opened up when a part of the North American Plate was torn away from the Oceanside / Camp Pendleton / San Onofre Area by the movement of the Pacific Plate about ten million years ago. That part of the North American Plate was carried north and rotated to form the western part of the Transverse Ranges, including the Channel Islands / Santa Barbara area. (See Tanya Atwater's geologic animation and Transverse/Peninsular Ranges Connections - Nine Lines Of Evidence For The Incredible Miocene Rotation.) That basalt flooded southern Orange County and southwest Riverside County, and eventually covered the nearly-flat landscape, producing an almost absolutely-flat landscape. See Santa Rosa Plateau Geology for more information.

The surface of the basalt had many small (~inch length) irregularities. Rain lingered in the small depressions, and the carbonic acid produced by dissolved carbon dioxide from the air weathered those depressions more than the surrounding small projections. Water runoff carried away most of the weathered basalt, deepening the depression.

The depressions grew with every rainfall, merging with neighboring small depressions over and over again over millions of years. Eventually, the surface was covered with large depressions ~0.5 mile wide, with relief of typically 100 feet elevation difference between the center of the depression and the rims. The Main Pool on the Vernal pool Trail on the Mesa de Colorado is the best place to see a depression with a nearly-intact rim. The small pools near the Vernal pool Trailhead have had their north and south rims removed by erosion; only their west and east rims are still intact.

When most of the basalt area was intact, perhaps in the period from two to ten million years ago, it is possible that the depressions contained permanent lakes, not vernal pools, and the Santa Rosa Plateau area was a Land of a Thousand Lakes, and not a vernal pool area. However, no one knows for sure what the fluvial regime was prior to the erosion of most of the basalt.

Once most of the basalt covering the entire area was removed by erosion, leaving the remnant basalt in mesas with steep sides, each of the remaining depressions was eventually deeply cut by stream erosion, initially in a single location. This is the first step of the same process that removed the surrounding basalt. This drastically lowered the water level in each depression, and resulted in the creation of vernal pools. See a cross-section through the Main Pool and its surrounding depression in How the Main Pool Forms and Disappears.

The vernal pool phase of these depressions lasts for only a brief geologic time; eventually, as the sides of the mesa get closer to the rims of the depressions, the stream outlet erodes deeply enough to completely drain the pools.

However, until that time, the stream outlet always stays just above the bottom of the vernal pool, even as both continue to lower due to erosion of the basalt. The basalt at the stream outlet is weathered just as fast as the rest of the pool perimeter since the area at the stream outlet is the perimeter of the pool.

The stream outlet can never get as deep as the bottom of the depression as long as the stream gradient is shallow since the erosive power of water is very small when there is little elevation difference. Hence the depressions are always able to store some water at their bottom, with the peak water level regulated by the height of the stream outlet. At the Santa Rosa Plateau, the pool depths are measured in inches, not feet.

That water has only two ways out: evaporation or percolation. Which is most important depends on the rate of those two processes.

The data I've collected over a decade shows clearly that percolation is the most important process. Without further rainfall, the pool depth declines by one inch per week, independent of the evaporation rate. The decline of one inch per week does not depend on season, the air or water temperature, or how windy it is.

For more information, including a detailed diagram of water flow, see How the Main Pool Forms and Disappears.

A general description of what happens every winter and spring is as follows. After a pool forms, fairy shrimp hatch from cysts that have survived complete desiccation through the heat of at least one summer, and often many summers. In only 40-50 days, they live their complete life cycle, and the population is back to only cysts. What is even more amazing is that the cysts are not eggs; cysts contain fully-formed baby fairy shrimp that are in very close to suspended animation! This is a very neat trick that we have yet to understand, and would probably have predicted would be impossible if it didn't exist.

Plants already begin to grow while water is in the pool, but with a few exceptions don't begin to bloom until the water is almost gone. Their development is thus delayed relative to plants in the surrounding landscape. Peak bloom outside of the vernal pool is typically mid-April; peak bloom in the vernal pool is typically mid-May. When the pool dries up completely, the plants complete their life cycle, with their seeds awaiting the rejuvenation of the pool.

If the vernal pool is significantly deeper in its center than near its edges, the life cycles of the plants will begin first at the edges, following the edge of the pool as it recedes. Thus concentric rings of flowers bloom successively, starting at the largest attained perimeter of the pool and subsequently shrinking toward the center. Often there is a succession of plants at a given spot, which creates concentric rings of flowers visible at optimum times. These are among the most breath-taking of pools.

Unfortunately, none of the pools at the Santa Rosa Plateau have enough depth variation to create concentric rings of flowers. In the past, there has been a purple outside ring from downingia blooms. Unfortunately, that ring has not been observed in recent decades due to a native grass, Paspalum distichum, crowding out the downingia, perhaps because of the removal of the cattle that grazed here until 1991. At best, near the end of the lifetime of the pool, tarweeds, grasses and popcorn flowers create a few large rings of muted color variations at the edge of the pool.

Around 2,000 vernal pools still exist in the coastal mesas of San Diego County, underlain by hardpan (caliche) which prevents drainage of seasonal rainfall. Many of those pools also support fairy shrimp and produce a ring of flowers. However, most of those pools are in military areas and closed to the public. The Main Pool at the Santa Rosa Plateau is the largest and best pool in southern California open to the public.

For more information about the Main Pool, see:

Links to my detailed observations of the vernal pool each year, including plots of the pool depth versus time, are given on the main page Field Guide to the Santa Rosa Plateau.

Go to:

Copyright © 1996-2007 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Updated 14 January 2008.