UCAY'ALI, a great river of South America, one of the chief head-waters of the Ama zon. It joins the Amazon from the s., in s. lat. 4' 40', and w. long. 73° 30', opposite the town of Nauta in Ecuador; but the whole course of the river is in Peru. It is the largest river that joins the Amazon above the Brazilian territory, and, on account of its length, has been regarded by some as the main stream of the Amazon, but at its mouth it is not above half the width of the Amazon. The Maranon and Huallaga from the s., with many smaller but still large rivers from the n., have united to form the Amazon. The sources of the Ucayali are in the Andes, Cuzco being situated on one of its feeders, which rises considerably further s.; while another has its rise on the western side of the Andes, to the n.w. of Lima, and after flowing southward for about 150 makes its way through a cross valley, and takes a northward course. Attention has of late been very strongly directed to the Ucayali as affording means of communication between the western parts of Peru and the Atlantic ocean. It was partially explored by the count de Castelnau and others in 1846, by lieut. Herndon and Mr. Gibbon of the U. S. navy in
1851, and more recently by an expedition sent out by the Peruvian government. It has been found to be navigable by steamers from its mouth to towns not far distant from Lima, 3,700 in. from the mouth of the Amazon. On the branch which comes from Cuzco there are falls and rapids, which form an impediment to navigation more than 100 m. below that city. The course of the Ucayali is winding, but generally northward. Without regard to any but its principal windings the length of its course is not less than 1100 miles. It receives many large branches. The name Ucayali is not given to any of its head-waters, the chief of which is the Tambo, formed by the junction of the Mantaro, a river which has its sources to the n.w. of Ulna, and the Apurimac, which comes from a more southern region. The greater part of the country through which the Ucayali flows is covered with forest; but it seems very suitable for colonization, if easy commu nication with the rest of the world were established, the soil being fertile, while the mountains abound in valuable minerals.