The Southampton Bay Wetland Natural Preserve makes up 70% of the park.
The Southampton mudflat formed eroded upriver silt and clay deposits exceeds 1,000 feet (300 m) thick.
The principal habitats here are brackish marsh, saltwater marsh and freshwater marsh.
This rare and endangered wetland ecosystem is covered with marsh plants such as salt grass (Distichlis spicata), pickleweed (Batis maritima), coyote bush (Baccharis pilularis) and soft bird's-beak (Cordylanthus mollis).
Bird’s-beak is an endangered gray-green annual herb in the snapdragon family.

Park mammals include the federally endangered northern salt marsh harvest mice (Reithrodontomys raviventris halicoetes).
Other mammals living in the park are coyote (Canis latrans), river otter (Lontra canadensis), muskrat (Ondatra zibethicus) and California golden beaver (Castor canadensis subauratus).
The beaver probably migrated from the Sacramento-San Joaquin River Delta in 2007.
Historically, before the California Fur Rush of the late eighteenth and early nineteenth centuries, the Delta probably held the largest concentration of beaver in North America.
It was California's early fur trade, more than any other single factor, that opened up the West, and the San Francisco Bay Area in particular, to world trade.
In 1840, explorer Captain Thomas Farnham wrote that "There is probably no spot of equal extent in the whole continent of America which contains so many of these muchsought animals."

Benicia SRA has been designated an Important Bird Area, providing habitat for endangered California clapper rails (Rallus longirostris obsoletus) and black rails (Laterallus jamaicensis).
Other uncommon species include Virginia rails (Rallus limicola), Suisun song sparrows (Melospiza melodia maxillaris) and salt marsh common yellowthroat (Geothlypis trichas sinuosa).
On their journey along the Pacific Flyway, many waterfowl winter in the park.
The Southampton Bay Wetland Natural Preserve makes up 70% of the park. The Southampton mudflat formed eroded upriver silt and clay deposits exceeds 1,000 feet (300 m) thick. The principal habitats here are brackish marsh, saltwater marsh and freshwater marsh. This rare and endangered wetland ecosystem is covered with marsh plants such as salt grass (Distichlis spicata), pickleweed (Batis maritima), coyote bush (Baccharis pilularis) and soft bird's-beak (Cordylanthus mollis). Bird’s-beak is an endangered gray-green annual herb in the snapdragon family.
Park mammals include the federally endangered northern salt marsh harvest mice (Reithrodontomys raviventris halicoetes). Other mammals living in the park are coyote (Canis latrans), river otter (Lontra canadensis), muskrat (Ondatra zibethicus) and California golden beaver (Castor canadensis subauratus). The beaver probably migrated from the Sacramento-San Joaquin River Delta in 2007. Historically, before the California Fur Rush of the late eighteenth and early nineteenth centuries, the Delta probably held the largest concentration of beaver in North America. It was California's early fur trade, more than any other single factor, that opened up the West, and the San Francisco Bay Area in particular, to world trade. In 1840, explorer Captain Thomas Farnham wrote that "There is probably no spot of equal extent in the whole continent of America which contains so many of these muchsought animals."
Benicia SRA has been designated an Important Bird Area, providing habitat for endangered California clapper rails (Rallus longirostris obsoletus) and black rails (Laterallus jamaicensis). Other uncommon species include Virginia rails (Rallus limicola), Suisun song sparrows (Melospiza melodia maxillaris) and salt marsh common yellowthroat (Geothlypis trichas sinuosa). On their journey along the Pacific Flyway, many waterfowl winter in the park.
Benicia State Recreation Area