The Sarstoon River is a Central American river that forms part of the international border between Belize and Guatemala. The source of the 111-kilometer-long river lies in Guatemala's Alta Verapaz Department. It flows eastward, serving as the border between the departments of Petén and Izabal before passing between Belize's Toledo District and Guat…The Sarstoon River is a Central American river that forms part of the international border between Belize and Guatemala. The source of the 111-kilometer-long river lies in Guatemala's Alta Verapaz Department. It flows eastward, serving as the border between the departments of Petén and Izabal before passing between Belize's Toledo District and Guatemala's Izabal Department on its way to the Caribbean Sea. Near its mouth lies Sarstoon Island, in Belizean territory.