"Despite their number and their importance, the rivers of this part of the country are subject to the common fate of all the higher affluents and sub-affluents of the Amazon--namely, a considerable diminution of their waters during the dry season, which renders the channels difficult and unreliable on account of the obstacles which accumulate at certain points. It is really only during the season of high water that steam navigation is easy and rapid; in general, on the Rios Beni and Madre de Dios, the steamboats run freely from December to May. From June and July the steamers encounter increasing numbers of obstacles and are exposed to the risk of a sudden fall of water, when the types of vessel peculiar to each river come into use, particularly from August to November. "During the months of July and August navigation is inconvenient on account of the expanse of mud, which has not as yet had time to harden, but is left uncovered by the falling waters on either bank, making landing a difficult matter. The reefs uncovered at low water and the entanglements of tree trunks disappear with the rise of the waters, and the steamers recommence running. "Descending the rivers during high water, navigation is both easy and rapid, but it is also rather dangerous on account of the swiftness of the current. It is difficult to estimate the time needed to navigate this or that river, as the day's journey, ascending or descending, varies according to the river, the amount of water in it, the kind of vessel employed, the amount of cargo carried, and the crew which steers or propels the vessel. "The vessels peculiar to the rivers of those regions which do not permit of steam navigation, and which are everywhere employed when the waters are low, are the _balsa_ and the _callapo_, each a species of raft; together with boats of various dimensions--_monterias_ or _batelon_, _egariteas_, _canoas_ or _pirogues_, the latter being dug-out canoes.