Ucayali, river, c.1,000 mi (1,610 km) long, formed by the confluence of the Apurímac and Urubamba rivers, E Peru, and flowing generally north through a mountain and jungle wilderness to the Marañón River, SW of Iquitos. It is a main headstream of the Amazon River; the headwaters of the Apurímac are the most distant from the Amazon's mouth. The Ucayali is navigable for its entire course by small craft. It is an important communications link for Iquitos.