The San Gregorio Creek Estuary, located on the central California coast, is a small but ecologically important system where the 15-mile (24-kilometer) San Gregorio Creek meets the Pacific Ocean. This estuary is intermittently closed by a sandbar that forms at the creek mouth, especially during dry periods. When the sandbar breaches—either naturally during high flows or through human intervention—it creates dynamic shifts in water levels, salinity, and nutrient cycling. These changing conditions shape the physical environment and influence the timing and availability of habitat for a wide range of species.

The estuary lies within a 42-square-mile (109-square-kilometer) watershed and supports diverse habitats, including marshes, riparian zones, and tidal pools. Species such as coho salmon and steelhead trout rely on the estuary as a critical transitional zone for rearing and migration. It also serves as a feeding ground for birds and a buffer that helps absorb and slow floodwaters during winter storms, reducing the risk of coastal flooding.

However, the estuary faces increasing pressure from upstream land use, water withdrawals, and habitat alteration, all of which affect its hydrology and ecological integrity. Ongoing water level monitoring helps track seasonal changes and breaching patterns, supporting restoration planning and long-term stewardship of this sensitive coastal system.