evidence: Essequibo River, river in east central Guyana, the largest river between the Amazon and the Orinoco. It rises in the Acarai Mountains on the Brazilian border and flows northward for approximately 630 miles (1,010 km) through savannas and forests to the Atlantic Ocean. It reaches the Atlantic Ocean 13 miles (21 km) west-northwest of Georgetown, the national capital. Its estuary, 20 miles (32 km) wide, is obstructed by islands and silt. With its chief tributaries, the Rupununi, Mazaruni, and Cuyuni, its system drains more than half of Guyana. The Essequibo is navigable by small ocean vessels to Bartica, 50 miles (80 km) inland, and by boats for long reaches between rapids. Its name is that of the old Dutch colony of Essequibo, settled from 1616 and one of the entities that joined together in 1831 to form British Guiana, now Guyana.