The Río Ucayali, together with Apurímac River, Ene River, and Tambo River, is today considered the main headwater of the Amazon River, totalling a length of 2,669.9 km from the source of the Apurímac at Nevado Mismi to the confluence of the Ucayali and Marañón Rivers:
The Ucayali was first called San Miguel, then Ucayali, Ucayare, Poro, Apu-Poro, Cocama and Rio de Cuzco. Peru has fitted out many costly and ably-conducted expeditions to explore it. One of them (1867) claimed to have reached within 380 km (240 miles) of Lima, and the little steamer "Napo" forced its way up the violent currents for 124 km (77 miles) above the junction with the Pachitea River as far as the Tambo River, 1240 km (770 miles) from the confluence of the Ucayali with the Amazon. The "Napo" then succeeded in ascending the Urubamba branch of the Ucayali 56 km (35 miles) above its union with the Tambo, to a point 320 km (200 miles) north of Cuzco.
The remainder of the Urubamba, as shown by Bosquet in 1806 and Castelnau in 1846, is interrupted by cascades, reefs and numberless other obstacles to navigation. Torres, who explored the Alto Ucayali for the Peruvian government, gives it a length of 186 miles, counting from the mouth of the Pachitea to the junction of the Tambo and Urubamba. Its width varies from 400-1200 m (1300 to 4000 feet), due to the great number of islands. The current runs from 5-6 km/h (3 to 4 miles an hour), and a channel from 20-50 m (60 to 150 feet) wide can always be found with a minimum depth of 1.5 m (5 feet). There are five bad passes, due to the accumulation of trees and rafts of timber. Sometimes enormous rocks have fallen from the mountains and spread over the river-bed causing huge whirlpools.