Flooding

A NASA satellite image of a flooded portion of the river.

Not all of the Amazon's tributaries flood at the same time of the year. Many branches begin flooding in November, and may continue to rise until June. The rise of the Rio Negro starts in February or March, and begins to recede in June. The Madeira River rises and falls two months earlier than most of the rest of the Amazon.

The average depth of the Amazon between Manacapuru and Óbidos has been calculated as between 20 to 26 metres (66 to 85 ft). At Manacapuru the Amazon's water level is only about 24 metres (79 ft) above mean sea level. More than half of the water in the Amazon downstream of Manacapuru is below sea level. In its lowermost section the Amazon's depth averages 20 to 50 metres (66 to 160 ft), in some places as much as 100 metres (330 ft).

The main river is navigable for large ocean steamers to Manaus, 1 500 kilometres (930 mi) upriver from the mouth. Smaller ocean vessels of 3 000 tons or 9 000 tons and 5.5 metres (18 ft) draft can reach as far as Iquitos, Peru, 3 600 kilometres (2 200 mi) from the sea. Smaller riverboats can reach 780 kilometres (480 mi) higher as far as Achual Point. Beyond that, small boats frequently ascend to the Pongo de Manseriche, just above Achual Point.

Exercise 11. Read the following text “Geography” and choose the right answer to the questions given below:

Geography

Meeting of Waters is the confluence of the Rio Negro (black) and the Rio Solimões (sandy) near Manaus, Brazil.

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.

Questions: