Essequibo (ĕsākēˈbō) [key], longest river of Guyana, c.600 mi (970 km) long, rising in the Guiana Highlands, S Guyana, and flowing generally N to the Atlantic Ocean. Most of the river's course is broken by rapids and waterfalls. There are many islands in its wide estuary. Bartica, the chief town on the river, is at the head of oceangoing navigation; gold, diamonds, and timber are moved on the river. With its tributaries the Essequibo drains more than half of Guyana.