Essequibo, river, central Guyana. The country's longest river, it rises in the Acarai Mountains, near the southern frontier with Brazil, and flows north for about 965 km (600 mi) to the Atlantic Ocean, near Georgetown, where it forms a wide estuary. It is navigable by large vessels for 80 km (50 mi) from its mouth. Beyond this point are numerous cataracts. The famous Kaieteur Falls (about 225 m/ 740 ft) is on the Potaro branch of the Essequibo.