The Pool on the Hill
There is a stretch of land along Sunrise Boulevard in east Sacramento County that appears, from one’s car window, to be absolutely inhospitable to life.
The streets nearby have names like Mechanical Drive, Recycle Road and Refinement Road. This is a light industrial area. Scrap yards sprawl along dozens of acres by the roadside.
Hang a right on Douglas Boulevard and the Cordova Shooting Range pops into view, with its rows of tan wooden shooting blinds and pick-up trucks and SUVs parked out front. A dead jackrabbit reposes on the dotted yellow line, but he is most likely a traffic fatality and not a shooting victim.
Hang a left onto Eagle’s Nest Road, and you will be entering the old Mather Field Air Force Base from the backside.
This stretch, for about two miles south along either side of Eagle’s Nest Road, is Mather Regional Park, which was given over to the county when the base closed in the early 1990s.
In winter, during the wettest part of the year, Mather park is basically a brown, lifeless stretch of fallow grassland, pocked here and there by a series of shallow pools that have filled with rainwater over the winter months. They are the kind of pools that a rancher might call hog wallows, all but useless from an agricultural perspective. They are also the kind of pools that devotees of off-road driving delight in aiming their trucks into.
But take a closer look into one of these pools. All along the bottom are tiny shoots of plants that have germinated during the cold, wet winter. Within a few weeks, this landscape will transform itself in remarkable ways.
Sometime in March, a careful observer will notice tiny signs of life. First, microscopic bacteria and protozoa, diatoms and algae begin to appear, too small for the naked eye to see.
But then tiny insects, the water flea and water beetle, show up, followed by specialized crustaceans.
It may be odd to think of shrimp living in a landlocked area that is bone dry most of the year, but all manner of shrimp thrive in these pools, having survived the scorching summer months in cysts laid a few inches under the pool floor. They hatch when the winter rains have filled the pools enough for them to thrive.
By mid-March, the pools are teeming with invertebrates, both shrimp and insects. Attracted by this sudden explosion of life, and food, it isn’t long before larger animals, frogs, snakes and mammals, appear. What was only weeks ago a seemingly dead pool, has quickly become a rich and complex food web of predators and prey. And each pool, even if separated from its neighbor by only a few yards, is a largely self-contained ecosystem.
This scale, the presence of entire food chains within a small area, makes the vernal pool an ideal classroom for students of ecology.
Indeed, every spring, the vernal pool complex at Mather Field becomes a sort of laboratory for Sacramento schoolchildren participating in a unique science program.
Eva Butler, a biologist and environmental consultant, is a volunteer for Sacramento Splash, a program that teaches fifth-graders about watersheds and ecology, using Mather Field as a classroom.
The Sacramento Splash program is a cooperative effort between the Sacramento County Regional Sanitation District, the county Department of Utilities, and the Water Resources Department, all of which are responsible for area water quality. Butler and other volunteers lead students in their investigation of the pools.
“This is a really good example of a clean water environment that is as it should be,” Butler says, as she and a group of animated fifth-graders kneel alongside of one pool.
The kids are on a sort of scavenger hunt—they have a list of species that they are supposed to find and then circle on a page.
They don’t find it difficult; within moments they are shouting and pointing and madly crossing things off their lists.
“Do you see the tadpole shrimp?” one boy yells, pointing out the reddish brown carapace—perhaps an inch long (it’s still young)—that completely covers the crustacean’s body, making it appear at first glance as if it were a tadpole.
Actual tadpoles are present as well, particularly those of the Pacific chorus frog. And an adult chorus frog, light green with a dark goggle-pattern around its eyes, basks in the shallow edge of the pool.
“I found detritus,” a little girl says, peering through the water at the bits of mysterious flotsam, the remains of plants and animals, the tiniest bits of the pool’s food web.
“I see a fairy shrimp,” exclaims another boy, eyeing a ghostly pale, almost translucent, twenty-two-legged crustacean.
The kids fill large specimen cups with pool water, and each comes up chock-full with minute wriggling and skittering life. They can dip their cups only because Butler has a permit from the U.S. Fish and Wildlife Department. Otherwise, this kind of “taking” borders on illegal, given that the fairy shrimp is federally listed as a threatened species and the tadpole shrimp is an endangered species.
Other students follow vole runs, small trails in the grass, perhaps four inches wide and four inches deep that meander for hundreds of yards, from pool to pool. The vole runs are paths worn by the California vole, which is similar to a mouse. The runs provide regular routes to food, from the edge of the pool and the small burrows that are found here and there. More important, the runs serve as secret runways that allow voles to escape unseen by predators above, especially the red-tail hawks that are abundant in the area, egrets, burrowing owls and the occasional coyote. The vole runs are well etched into the landscape; thousands of trips, made by successive generations of voles, must have been required to form them.
The students trundle their specimens back to a classroom at the nearby Mather Heights Elementary School. The school has dedicated the classroom to the Sacramento Splash program. Banks of microscopes line two long tables; muddy pairs of shoes line one inside wall.
This class happens to be studying the pools in the first of its three distinct phases. This is the wet phase, after the winter rains fill the pools and the first critters start to appear. Within a few weeks, as the rains taper off, the next phase will begin. As the pools dry, an array of wildflowers will begin to appear, and flora will trump fauna’s prominence in the vernal pool landscape.
Even while the pools are full, however, the flowers are preparing for their ascent.
Along the bottom of each pool, under the water, plants carpet the bottom.
Vernal pool buttercups occupy the bottom of one pool. Their leaves are upside-down teardrops, each cradling a pearl-like bubble of air.
Despite its proximity to the industrial area along Douglas Boulevard, despite decades of use as a military base, the vernal pool area is still remarkably pristine. It is, as Butler said, as it should be. And it is, for the most part, as it has been for hundreds of thousands of years.
“There are so few places in the Central Valley where students can safely explore the diversity of life in an aquatic ecosystem that hosts much the same life today as it did 100,000 years ago,” Butler explains.
The vernal pool is a quickly vanishing window into the past, to a time long before humans existed in California.
About two-and-a-half million years ago, shortly after the Sierra Nevada mountains rose in the east, melting glaciers fed a number of now-defunct rivers that once crisscrossed the Central Valley. What might be called an ancestral American river washed through this area, depositing sediment from the mountains. At the same time, the ancient river scoured the earth, forming a series of depressions and raised mounds. Over hundreds of thousands of years, this river changed its course three times, each time farther to the north and west and each time farther down slope.
The river has long since vanished. But the land here still bears its mark. The end result is that Mather Field is separated into three distinct terraces, the upper in the north, followed by a middle terrace and lower terrace farthest to the northwest, each progressively lower in altitude.
If you were to push a broomstick into the ground, you would only break through about two or three feet of soil before you hit the underlying hardpan.
Volcanic eruptions at what are now Sutter Buttes to the north and the Long Valley volcano at what is now Mammoth Lake to the west deposited ash heavy in silica and iron components, which percolated down through the alluvial soil to form a layer of hardpan that is found throughout the Sacramento region.
But because the hardpan is impermeable to water, pools tend to form in the winter wherever there are depressions.
The ground is unaccommodating to most tree species because the hardpan doesn’t allow for much root growth. But scores of other plant species, especially grasses and wildflowers, have adapted to the unique vernal pool environment. In all, about 60 different plant species grow only in vernal pools. Most are present at Mather.
Aside from the pools and their underlying hardpan, the areas contain another unique, and somewhat more mysterious, landform. “Mima mounds,” one or two feet high and four to six feet in diameter, and usually covered in grasses, dot the landscape. Nobody is sure why the mima mounds have formed, although there are several theories.
The most widely held explanation is that the mounds were formed by wind and running water, the very same force that carved out the pools. Under the “selective erosion” scenario, the mounds are pockets of more erosion-resistant soils that remained after looser material had been swept away along ancient creeks.
This theory doesn’t quite hold the satisfaction of perhaps the oldest human attempt to explain the mima mounds. Thousands of years ago, as the Miwok Indian myth goes, Central Valley tribes were under constant attack by marauding Nevada tribes. Miwok women took up their burden baskets, filled them with earth and then piled them on top of each other to form an earthen wall, too high and too strong for the eastern tribes to surmount. Thus the Sierra Nevada mountain range was formed. When the work was done, and their enemies were repelled, the exhausted squaws dropped their empty baskets wherever they stood and walked away. Those baskets, according to legend, are the mima mounds.
Because of the filling and drying cycle of the vernal pool wetland, plant species have adapted in unusual ways—ways that can be quite striking to the eye.
Vernal pool flowers tend to grow in well-defined rings around the pools. A typical pool could have anywhere from two to four concentric rings of flowers by April.
Meadowfoam, a greenish white flower, encircles a ring of tidy tips, bright yellow with white edges. Farther toward the middle, pinkish checkerbloom tightly hug a cluster of goldfields, the most spectacular of the vernal pool flowers. At the peak of the flowering season, around the week of April 20, the concentric flower can be stunningly beautiful.
Scientists speculate that some species like to be wetter for a longer period of time, and these tend to cluster near the center of the pools. Others thrive best with shorter periods of submersion, and these tend to grow out near the edges.
At least that’s what meets the eye. Why the flowers grow in these distinctive rings is still largely a matter of broad speculation. Years of experimentation and observation remain to tease out just which ecological processes drive the ring patterns, whether they be soil type, slope of ground or the amount of water, or other, thus far unknown, processes.
Nancy Emery, a 25-year-old grad student at UC Davis, has decided to do her doctorate work in evolutionary ecology at the Mather Field vernal pools.
She says that the close proximity of so many subtly different habitats in one area allows for elaborate experiments that might not be possible in other ecosystems.
Emery spends days in the field, methodically moving plants from one area of the pool to another, carefully noting just which species belong where. The pay off, research-wise, will be determining if flowers grow where they are “not supposed to,” or if they can’t survive even a few feet from where they are normally found. Having determined that, the next question will be why. Then, an even broader question will come into play: why does one species live at one pool, but is completely absent from a pool only a few yards away?
“In terms of research, this is a goldmine,” says Emery.
The truth is, she says, that so little is known about vernal pool wetlands that there is a great opportunity to add to the evolutionary and ecological literature, and, in doing so, bring us closer to answers for the big questions: how do new species form, what role does competition play in an ecosystem?
And as vernal pool wetlands go, Mather is exceptional.
“I looked around at some other vernal pools locally and nothing compares,” says Emery.
Vernal pools in California face a great deal of competition themselves.
The grasslands surrounding the pools are encroached upon by a whole host of non-native plants that threaten to out-compete the specialized native plants.
Erodium, also called scissor plant or filaree, appeared a couple of hundred years ago, born by the livestock of Spanish colonists, especially sheep with their matted wool just perfect for transporting foreign seeds. Medusa head and star thistle are two others that thrive here in the absence of their natural competitors. Indeed, erodium is more common in California than it is in its native Europe.
Aside from the natural (albeit non-native) enemies of the vernal pool ecosystem, this area has man-made enemies as well.
Statewide, only about 10 percent of all vernal pools that once existed still remain. Of these, two-thirds are protected by the federal Endangered Species Act. The act, however, does not guarantee that existing pools be left alone. They can be filled or otherwise destroyed by the property owner who is willing to jump through enough bureaucratic hoops.
At Mather, urbanization and mining are the main threats to the existing vernal pools.
Particularly on the lower terrace, geologically the youngest area, the alluvial processes have made Mather a rich resource for gravel. Gravel extraction is a major industry in the Sacramento area, and, not surprisingly, various gravel mining interests have their eye on what they believe is millions of dollars worth of the rock.
The mining company Teichert Aggregates has mining operations just to the south of the lower terrace, which runs for about two miles along the runway of the Mather Field Airport. This is the most highly concentrated area of pools, and biologically the most unique area in the Mather complex.
Teichert and two other companies have long expressed interest in mining the area, and the county is in the process of determining just what the value of the underlying gravel bed is.
But Sacramento County’s director of economic development, Paul Hahn, says that the county is far from making up its mind about whether to allow gravel mining on the property. The county is also considering expansion of the existing airport, which could impact at least some of the pools.
In the southeast corner of the old base, the county is considering selling another 500 acres off for development. That area, said Hahn, has a high concentration of pools, but they aren’t as biologically valuable as the pools near the runway. The southeast corner also abuts an area outside the base property that is already slated for heavy residential development.
Because the conversion of Mather has to pay for itself, Hahn says it is almost inevitable that some of the vernal pools will be lost. The trick will be coming up with ways to preserve the best pools.
“To protect the vernal pools, we may have to destroy some vernal pools,” says Hahn.