The Altamaha River orginates at the confluence of the Ocmulgee River and Oconee River, at the three way county boundary between Montgomery, Jeff Davis, and Wheeler Counties. The Altamaha then flows eastward, and a little southward, for 137 miles to empty into the Atlantic Ocean near Darien, Georgia. Its largest tributary is the Ohoopee River, which joins the Altamaha on the southern border of Tattnall County. The Altamaha River watershed forms the largest freshwater river system east of the Mississippi River. The Altamaha River is Georgia's largest (though not longest) river.