=The Xingú=, the next river, with a length of 1260 miles is the first true tributary of the Amazon on the south. This also flows nearly north with many rapids and at last a fall, Itamaracá, at the head of navigation, 105 miles from its mouth. Near this the river expands into a great lake which communicates with the Amazon by many natural channels. =The Tapajós=, 1158 miles long, entering the Amazon about 500 miles above Pará, rises near Diamantino on the plateau, and flows through a long, hot, unhealthful valley. One hundred and eighty-eight miles are navigable, the lower 100 of these being 4-9 miles wide and throughout much of its course very deep. Along the lower river valley are bluffs 300-400 feet high. =The Madeira=, entering 870 miles above Pará, almost rivals the Amazon in volume. In the rainy season, during which it rises 50 feet, the largest ocean steamers may ascend 665 miles to the falls of Santo Antonio and the Madeira-Mamoré Railway; from June to December it is navigable for vessels of 5-6 foot draft only. Tributary to the Madeira on the east is the river formerly called the Duvida, first completely traversed by Colonel Roosevelt and his party and now named in his honor Rio Roosevelt. The Madeira which has 90 tributaries and a basin of over 600,000 square miles is formed a little higher up by the junction of the Beni and Mamoré, both of which streams rise close to the head waters of the Paraguay. From Santo Antonio south, the Madeira-Mamoré is obstructed for 263 miles by a series of rapids and cataracts, the Madeira Falls, where the Beni comes in, presenting a vast display of whirlpool and boiling torrents. Above Santo Antonio the drainage is received of the southwest slope of the State of Matto Grosso, the north slope of the Chiquitos Sierras, and the northeast slope of the Andes from Santa Cruz de la Sierra in Bolivia to Cuzco in Peru. The most important of the affluents are the Guaporé, Baurés, Itonamas, Mamoré, Beni, and Madre de Dios. The almost level Mojos and Beni plains are said to rival if not to exceed in fertility the Nile Valley; they are the most healthful and most inviting grazing and agricultural regions in the Amazon Basin, which has an area about equal to that of France and Spain. However, 35,000 square miles are flooded 2-3 feet, three or four months every year. =The Purús=, over 2000 miles in length, is a very sluggish stream parallel to the Madeira, in the great depression between the Brazilian plateau and the Andes. A peculiar feature is five parallel canals coming into it from the Amazon northwest at almost regular intervals, making five low islands; and nearer to the great river are three more. The Purús is navigable for light steamers 1648 miles five months in the year; for 800 miles its depth is not less than 45 feet. The lands are subject to inundation, the river at times rising 50 feet. =The Juruá= is a similar river, navigable for 1133 miles.