High Quality Content by WIKIPEDIA articles! The Ucayali River (Río Ucayali) arises about 110 km (70 miles) north of Lake Titicaca, in the Arequipa region of South America. The Amazon River takes its name close to Nauta city (100 Km away from Iquitos), in the confluence among Ucayali and Marañón rivers. The Ucayali, together with Apurímac River, Ene River, and Tambo River, is today considered the main headwater of the Amazon River, totaling a length of 2,669.9 kilometres (1,659.0 mi) from the source of the Apurímac at Nevado Mismi to the confluence of the Ucayali and Marañón Rivers.