Ucayali River, a river in eastern and northern Peru, and one of the main headstreams of the Amazon River. The Ucayali and its tributaries drain much of the Peruvian Andes. Flowing northward, it joins the River above Iquitos and forms the Amazon. The Ucayali is 1,200 miles (1,900 km) long. About half its length can be navigated by riverboats.