The San Gregorio Creek Estuary, located on the central California coast, is a small but ecologically significant system where the 15-mile (24-kilometer) San Gregorio Creek meets the Pacific Ocean. Covering an area that fluctuates seasonally, this intermittently-closed estuary is shaped by the periodic formation of a sandbar at its mouth, leading to changes in water levels, salinity, and nutrient dynamics. The estuary lies within a 42-square-mile (109-square-kilometer) watershed, supporting diverse habitats for species like steelhead trout and coho salmon, which rely on these waters for critical rearing and feeding grounds.

The estuary also acts as a natural flood buffer, helping to manage coastal flooding during high flow events. However, it faces challenges from upstream development, water extraction, and habitat alteration, impacting its ecological health. Ongoing monitoring and restoration efforts are essential to preserve the unique dynamics and biodiversity of this valuable coastal ecosystem.