The northern slope of this great plateau is drained by the AraguayaTocantins, Xingu, Tapajos and Guapore-Mamore-Madeira, which flow northward, and, except the first, empty into the Amazon; the southern slope drains southward through a multitude of streams flowing into the Parana and Paraguay.
Add to these the eroded river basins of the Xingu, Tapajos and Guapore on the north and west, the Paraguay on the south-west, and the scores of smaller rivers along the Atlantic coast, and we may have some conception of the agencies that have been at work in breaking down and shaping this great table-land, perhaps the oldest part of the continent.
It includes the river basins of the Tocantins-Araguaya, Xingu, Tapajos, and the eastern tributaries of the Guapore-Madeira.
The lower river valleys of the Tocantins-Araguaya, Xingu, Tapajos and Paraguay are essentially tropical, their climate being hot and humid like that of the Amazon.
The Tapajos, running through a humid, hot and unhealthy valley, pours into the Amazon 500 m.
Near this place a number of streams unite to form the river Arinos, which at latitude io° 25' joins the Juruena to form the Alto Tapajos, so called as low down as the Rio Manoel, entering from the east.
The lower Arinos, the Alto Tapajos and the Tapajos to the last rapid, the Maranhao Grande, is a continuous series of formidable cataracts and rapids; but from the Maranhao Grande to its mouth, about 188 m., the river can be navigated by large vessels.
Wide and much of it very deep. The valley of the Tapajos is bordered on both sides by bluffs.
Then they bend to the south-west, and, abutting upon the lower Tapajos, merge ' Igapo is thus the name given to the recent alluvial tracts along the margins of rivers, submerged by moderate floods, whereas vargem is the term used for land between the levels of moderate and high floods, while for land above this the people use the term terra firma.
On the south side, from the Tapajos to the river Madeira, the banks are usually low, although two or three hills break the general monotony.
The government of Brazil, constantly pressed by the maritime powers and by the countries encircling the upper Amazon basin, decreed, on the 31st of July 1867, the opening of the Amazon to all flags; but limited this to certain defined points - Tabatinga, on the Amazon; Cameta, on the Tocantins; Santarem, on the Tapajos; Borba, on the Madeira; Manaos, on the Rio Negro; the decree to take effect on the 7th of September of the same year.