The sand bar at Rodeo Lagoon's mouth is not static. During winter storms, waves can breach the bar, reconnecting the lagoon directly to the Pacific. In calmer months, the bar rebuilds, sealing the lagoon and allowing freshwater inputs from the surrounding watershed to gradually reduce salinity. This cycle of connection and isolation gives the lagoon a dynamic character that distinguishes it from both the ocean and inland freshwater bodies. The water level, salinity, and species composition fluctuate with the seasons, making the lagoon a natural laboratory for understanding how coastal ecosystems respond to changing conditions.

The lagoon and its margins support a variety of wildlife. Shorebirds and waterfowl use the lagoon for feeding and resting during migration along the Pacific Flyway. Great blue herons stalk the shallows. The surrounding vegetation -- a mix of coastal scrub, wetland plants, and riparian species along the inlet creeks -- provides habitat for amphibians, reptiles, and small mammals. The red-legged frog, a federally listed species, has been documented in the watershed. The lagoon's ecological value derives precisely from its in-between nature: it is neither ocean nor pond, neither permanent nor ephemeral, but something more complex and productive than either.

The lagoon sits at the bottom of a valley surrounded by the steep, grassy ridges of the Marin Headlands, two miles west of the Golden Gate Bridge. Former military buildings from the headlands' decades as a coastal defense installation line the roads nearby. The Marine Mammal Center, which rehabilitates injured seals and sea lions, operates in a former military structure above the lagoon's western shore. Trails connect the lagoon to Rodeo Beach, the Coastal Trail, and the headlands' network of ridgetop paths. The setting is remarkable: a quiet body of water in a folded landscape, minutes from one of the world's most photographed bridges, producing the sensation of remoteness in the middle of a metropolitan area of seven million people.