[1] The St. Marks River begins in eastern Leon County, Florida and flows 36 miles (58 km) through Leon and Wakulla counties into Apalachee Bay, an arm of the Gulf of Mexico.

A few miles south of its source the St. Marks passes under a natural bridge at Natural Bridge Battlefield Historic State Park and then disappears underground to become a subterranean river for about one-half mile.

The San Marcos de Apalache Historic State Park and St. Marks lighthouse are located near the mouth of the river, while the City of Tallahassee's Sam O. Purdom Generating Plant is located on the river in St. Marks.

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