section_path: Course/The Lower Amazon
section_name: The Lower Amazon
paragraph_text: The Lower Amazon
  
  The Lower Amazon begins where the darkly colored waters of the Rio Negro meets the sandy-colored Rio Solimões (the upper Amazon), and for over 6 km (3.7 mi) these waters run side by side without mixing. 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 10% 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 km (1,900,000 sq mi), and, below, only about 1,000,000 km (390,000 sq mi) (around 20%), exclusive of the 1,400,000 km (540,000 sq mi) of the Tocantins basin. The Tocantins River enters the southern portion of the Amazon delta.
  
  In the lower reaches of the river, the north bank consists of a series of steep, table-topped hills extending for about 240 km (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, a 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.