A modern definition of the border initially claimed by the United States begins at the mouth of Sabine Pass on the Gulf of Mexico (coord), thence up the Sabine River to its source (coord), thence due north (a distance of about 400 meters along meridian 96°12'58"W) to the southern extent of the Red River drainage basin (coord), thence westward along the southern extent of the Red River drainage basin to the tripoint of the Red River, Arkansas River, and Brazos River drainage basins (coord), thence northwestward along the southern extent of the Arkansas River drainage basin to the tripoint of the Mississippi River, Colorado River, and San Luis drainage basins (coord), thence northward along the Continental Divide of the Americas to the tripoint of the Mississippi River, Colorado River, and Columbia River drainage basins (on Three Waters Mountain) (coord).