The Estero Americano watershed covers 49 square miles. It forms a portion of the northern boundary between Marin and Sonoma counties where it drains into Bodega Bay.
The Estero Americano is a coastal estuary at the base of Americano Creek, the estuary's sole tributary.
In some years, a seasonal sand bar at the mouth restricts tidal exchange. When the mouth is open, the tidal influence ranges up to 4 miles upstream.