Skip to content Skip to navigation




Bordered on the east by the high Sierra Nevada and separated from the Pacific Ocean by the Coast Range mountains, the San Joaquin Valley is 250 miles long and 50 miles wide, and the flat, open landscape includes parts of eight counties.

The San Joaquin River, the Valley’s namesake, runs the length of the region north from the Tulare Lake Basin. This river is fed by the Merced, Tuolumne, Stanislaus, Mokelumne and Cosumnes rivers. Dams and reservoirs for agricultural irrigation and domestic drinking water suppplies have dramatically reduced the natural flow of the San Joaquin River and its tributaries.

As of 2011, 3.9 million people and more than 100 ethnic groups live in the San Joaquin Valley.  According to the California Department of Finance, the population will increase 131 percent by 2050 – the fastest increase in the state. Much of the Valley’s population is clustered in major cities, many dating from the late 1800s when they sprang up during construction of the Central Pacific Railroad. Those communities — Stockton, Modesto, Merced, Fresno and Bakersfield — are part of a string of urbanization along Highway 99, the region’s major intercity corridor. The population here is 5 percent younger than the state average.

The Merced Vernal Pools & Grassland Reserve sits at the edge of urbanization and agricultural fields, about five miles from downtown Merced.

Eastern Merced County includes a remarkably intact section of an alluvial terrace landscape formed along the western base of the Sierra Nevada. At first glance, the region and the site appear to consist of a relatively homogeneous mix of annual grasslands and vernal pools distributed across undulating slopes and occasional low hills. However, on more careful study, one finds this landscape to be quite varied in its soils, geology, and biology. Taken as a whole, this region can provide us with insight into the physical genesis and evolutionary history of a large region of California, including areas well beyond the boundaries of eastern Merced County.


The most striking habitats within the Reserve are the vernal pools. As seasonally inundated wetlands, the vernal pools require specific combinations of geology, soils, slope, and climate to exist.  The entire area encompasses one of the largest, least fragmented vernal-pool-grassland environments in the world. Some of the largest and most diverse sets of vernal pools found anywhere in California occur in this area. The area is defined by hundreds of small watersheds that support and create a dense vernal-pool complex, including the habitat for the extremely rare Conservancy Fairy Shrimp (Branchinecta conservatio). The grasslands of eastern Merced County occur within a region where precipitation falls only during the cool part of the year (November through May). This leads to a high diversity of annual flowering plants.

Eastern Merced County is home to some of California’s rarest, most endangered animals and plants. Each species has specific adaptations that allows it to survive in these sparsely vegetated lands that receives only about 13 inches of rain in a year. The survival of many of these rare and endangered organisms is linked to the vernal pools -  ephemeral wetlands that fill seasonally with rainfall and persist for only three to five months before drying. These pools are considered one of the most threatened ecosystems in California. Most have been lost as agriculture expanded across the Central Valley and to road building and urbanization.  Vernal pools are still found in widely-scattered areas throughout California, but they reach their greatest density and development on the alluvial terraces formed along the base of the Sierra Nevada foothills. Here, the landscape and weather are ideal for their development. Other uniquely characteristic habitats within this terrace landscape region include volcanic and sandstone rock outcrops, seasonally inundated clay flats, and certain geologically influenced microhabitats within the annual grasslands. The Vernal Pools & Grassland Reserve is characterized by unique globally-significant habitats that create important opportunities for teaching and research.

The site lies within the watersheds of the Black Rascal, Fahrens, and Cottonwood creeks, which flow generally southwest to Bear Creek and the San Joaquin River. Elevations vary from 250 to 570 feet, and the topography is flat to moderately rolling with distinct hills along the northeast border and 15 m deep ravines along some creeks. Some of the oldest soils in North America (the China Hat soil formation) are found in the high regions to the northeast.

Dominant plant species in the vernal pools include coyote thistle (Eryngium castrense), goldfields (Lasthenia fremontii), downingia (Downingia bicornuta), popcorn flower (Plagiobothrys acanthocarpus, P. stipitatus), woolly marbles (Psilocarphus brevissimus var. brevissimus), white meadowfoam (Limnanthes alba), annual hairgrass (Deschampsia danthonioides), and foxtail grass (Alopecurus saccatus). Special-status plant species include succulent owl’s-clover (Castilleja campestris var. succulenta), Hoover’s spurge (Chamaesyce hooveri), Colusa grass (Neostapfia colusana), San Joaquin Valley Orcutt grass (Orcuttia inaequalis), hairy Orcutt grass (Orcuttia pilosa), and Greene’s tuctoria (Tuctoria greenei).

According to John Dittes and Josephine Guardino (in: Volmer, J. 2002. Wildlife and Rare Plant Ecology of Eastern Merced County’s Vernal Pool Grasslands), this area represents a critical geographical area for the long-term, statewide conservation of many rare plants, especially the Orcuttieae grasses (Colusa grass, San Joaquin Valley Orcutt grass and Greene’s tuctoria), pincushion navarretia (Navarretia myersii), succulent owl’s-clover, Hartweg’s golden sunburst (Pseudobahia bahiifolia), spiny-sepaled button celery (Eryngium spinosepalum), Hoover’s calycadenia (Calycadenia hooveri) and Merced phacelia (Phacelia ciliata). Shining navarretia occurs as a disjunct population at the edge of its geographical range in eastern Merced County. This region is also very important for the Orcuttieae grasses in terms of potential undiscovered occurrences, and the opportunity for colonization of presently unoccupied, large vernal pools and stockponds.

Common upland native forbs include virgate tarweed (Holocarpha virgata), Fitch's Spikeweed (Hemizonia fitchii), vinegarweed (Trichostoma lancelatum), Turkey mullein (Eremocarpus setigerus), and prickly lettuce (Lactuca serriola).

Like much of California, the grasslands of eastern Merced County have undergone a large-scale replacement of native plants by European annual species over the past 150 years. Most of the grasses that now cover the lands are not native to this area. Brilliant green in winter and early spring, the grasslands become gold and brown from May to October in response to the lack of rain, an annual occurrence in this area. Since the 1800s, the grasslands here have been used for cattle or sheep grazing, a leading agricultural activity. The only building in or near the Reserve is a 100 year-old barn located 0.75 miles east of the main campus (directly adjacent to the Reserve), which marks a site where families lived and ranched decades ago.

Links to many relevant publications and studies of vernal pools are found at