river in western Africa, rising north of the Nimba Range in Guinea and flowing south to form more than half of the Liberia–Côte d'Ivoire border. It enters the Gulf of Guinea 13 miles (21 km) east of Harper, Liberia, after a course of 320 miles (515 km). With its major tributaries (the Duobe and the Hana), it drains an area of 11,670 square miles (30,225 square km). Named by 15th-century Portuguese navigators for the cavalla (horse mackerel) found at its mouth, it is navigable to Nyaake, 50 miles (80 km) upstream.