At some points the river divides into anabranchs, or multiple channels, often very long, with inland and lateral channels, all connected by a complicated system of natural canals, cutting the low, flat igapó lands, which are never more than 5 metres (16 ft) above low river, into many islands.From the town of Canaria at the great bend of the Amazon to the Negro, only very low land is found, resembling that at the mouth of the river. Vast areas of land in this region are submerged at high water, above which only the upper part of the trees of the sombre forests appear. Near the mouth of the Rio Negro to Serpa, nearly opposite the river Madeira, the banks of the Amazon are low, until approaching Manaus, they rise to become rolling hills. At Óbidos, a bluff 17 m (56 ft) above the river is backed by low hills. The lower Amazon seems to have once been a gulf of the Atlantic Ocean, the waters of which washed the cliffs near Óbidos.
Only about ten percent of the Amazon's water enters downstream of Óbidos, very little of which is from the northern slope of the valley. The drainage area of the Amazon Basin above Óbidos city is about 5,000,000 square kilometres (1,900,000 sq mi), and, below, only about 1,000,000 square kilometres (390,000 sq mi) (around 20%), exclusive of the 1,400,000 square kilometres (540,000 sq mi) of the Tocantins basin. The Tocantins River enters the Amazon very close to its mouth.
In the lower reaches of the river, the north bank consists of a series of steep, table-topped hills extending for about 240 kilometres (150 mi) from opposite the mouth of the Xingu as far as Monte Alegre. These hills are cut down to a kind of terrace which lies between them and the river.
On the south bank, above the Xingu, an almost-unbroken line of low bluffs bordering the floodplain extends nearly to Santarém in a series of gentle curves before they bend to the southwest, and, abutting upon the lower Tapajós, merge into the bluffs which form the terrace margin of the Tapajós river valley.
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