San Antonio Creek springs from Antonio Mountain in Marin County and runs north into Chileno Valley. Turning east-southeast, it begins to define the county line. It passes under Chileno Valley Road and Point Reyes-Petaluma Road, then parallels San Antonio Road eastward to U.S. 101. It crosses under U.S. 101 to its confluence with the Petaluma River near Hog Island in the wetlands south of Petaluma, California.
The U.S. 101 bridges are each two lanes wide and long. The northbound bridge was built in 1929 and reconstructed in 1979. The southbound bridge was built in 1947. There is also a bridge at San Antonio Road north of U.S. 101; built in 1917, it is a concrete tee beam long. Marshall Petaluma Road crosses the creek on a concrete tee beam built in 1929. Point Reyes Petaluma Road crosses on a concrete continuous slab built in 1964.