The Huallaga River (also known as the Guallaga and Rio de los Motilones), which joins the Amazon River to the west of the Ucayali, rises high among the mountains, in about 10 degrees 40 minutes southern latitude, on the northern slopes of the celebrated Cerro de Pasco.
For nearly its entire length it is an impetuous torrent running through a succession of gorges. It has forty-two rapids, its last obstruction being the Pongo de Aguirre , named for the rebel conquistador Lope de Aguirre who passed there. To this point, 140 miles from the Amazon, the Huallaga can be ascended by large river steamers.
Between the Huallaga and the Ucayali lies the famous "Pampa del Sacramento ," a level region of stoneless alluvial lands covered with thick, dark forests, first entered by Christian missionaries in 1726. It is about 300 miles long, from north to south, and varies in width from 40 to 100 miles. Many streams, navigable for canoes, penetrate this region from the Ucayali and the Huallaga. It is still occupied by hunter-gatherer tribes.
