section_path: Discharge/Flooding
section_name: Flooding
paragraph_text: Flooding
  
  Not all of the Amazon's tributaries flood at the same time of the year. Many branches begin flooding in November and might 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 river.
  
  The depth of the Amazon between Manacapuru and Óbidos has been calculated as between 20 and 26 m (66 and 85 ft). At Manacapuru, the Amazon's water level is only about 24 m (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 m (66 to 164 ft), in some places as much as 100 m (330 ft).
  
  The main river is navigable for large ocean steamers to Manaus, 1,500 km (930 mi) upriver from the mouth. Smaller ocean vessels below 9000 tons and with less than 5.5 m (18 ft) draft can reach as far as Iquitos, Peru, 3,600 km (2,200 mi) from the sea. Smaller riverboats can reach 780 km (480 mi) higher, as far as Achual Point. Beyond that, small boats frequently ascend to the Pongo de Manseriche, just above Achual Point in Peru.
  
  Annual flooding occurs in late northern latitude winter at high tide when the incoming waters of the Atlantic are funnelled into the Amazon delta. The resulting undular tidal bore is called the pororoca, with a leading wave that can be up to 7.6 m (25 ft) high and travel up to 800 km (500 mi) inland.