The Merced Vernal Pools and Grassland Reserve protects hundreds of ephemeral pools and swale wetlands spread across a remarkably intact alluvial terrace. Located in the foothills of the San Joaquin Valley, this rolling grassland contains perhaps the densest concentration of vernal pools in the West. China Hat Ridge has 2 to 4 million-year-old soils that are some of the most ancient continuously exposed soils in North America. Mima mounds—low, flattened, domelike formations composed of loose, unstratified, and often gravelly sediment—are abundant in some areas of the reserve. Non-native, European grasses dominate the landscape; the handful of willow and fig trees are found by cattle stock ponds or the creek. Intermittent Black Rascal Creek runs for 4.7 km (3 miles) along the southern boundary and cuts a channel up to 15 meters (45 feet) deep. The Reserve is adjacent to the UC Merced campus, making it especially accessible to faculty and students.
The area has poor clay soils and a meager water supply. Yet this harsh land is ideal for the formation of vernal pools. A layer of impermeable hardpan causes water to collect at the surface during the November to April rainy season. Vernal pools are a threatened ecosystem in California; only a small fraction of the state’s original pools still remain. With rain, the pools come alive with aquatic life, and brilliant, showy wildflowers bloom around their edges.
The vernal pools are home to two species of endangered fairy shrimp (Branchinecta conservatio, B. lynchi) and the endangered vernal pool tadpole shrimp (Lepidurus packardi). The mid-valley fairy shrimp (B. mesovallensis) has a very narrow distribution in the Central Valley but is not listed at present. California tiger salamanders (Ambystoma californiense), another federally endangered species, live underground here most of the year, but migrate to pools in the rainy season to reproduce. Plants include more than 25 endemic, rare and protected species. Fifty-seven bird species have been documented on the reserve, including Swainson’s hawks, Ferruginous hawks, and burrowing and short-eared owls. Immense flocks of horned larks sweep across the grassy hills in winter. The mammal community consists of at least nine species, including the American badger, while the endangered San Joaquin kit fox (Vulpes macrotis mutica) has been documented on ranches adjacent to the Reserve.
Since the mid-1800s, reserve lands have been grazed by sheep and cattle. The Reserve is currently leased to an organic dairy which grazes cattle for about half the year. The cattle help maintain the viability of the vernal pools. By consuming invasive European grasses, the cattle reduce soil water loss that could shorten the hydroperiod of the pools.