Its principal tributary is the Cotoxé River (or northern branch), which also arises in Minas Gerais near the source of the southern branch, in an area called "Alto São Mateus".[3] The two come together near the village of Lagoa at 18°39′02″S 40°05′37″W﻿ / ﻿18.65056°S 40.09361°W﻿ / -18.65056; -40.09361, and the São Mateus River flows 65 more kilometers, through the city of São Mateus, and empties into the South Atlantic Ocean near the town of Conceição da Barra, in the state of Espírito Santo. In addition, other rivers such as the São Domingos, the Santana, the Mariricu, and the Rio Prêto are part of the São Mateus River's hydrological system.[3]