yet well out of sight of land in what otherwise seemed to be the open ocean. The Atlantic has sufficient wave and tidal energy to carry most of the Amazon's sediments out to sea, thus the Amazon does not form a true delta. The great deltas of the world are all in relatively protected bodies of water, while the Amazon empties directly into the turbulent Atlantic. There is a natural water union between the Amazon and the Orinoco basins, the so-called Casiquiare canal. The Casiquiare is a river distributary of the upper Orinoco, which flows southward into the Rio Negro, which in turn flows into the Amazon. The Casiquiare is the largest river on earth that links two major river systems, a so-called bifurcation. Discharge Average discharge at the estuary; Period from 2003 to 2015: Average discharge at Óbidos gauge station; Period from 1969 to 2018: Average discharge (Q - 173,000 m3/s) and sediment load (S - 754 x 106 ton/year) at Óbidos gauge station (period from 1996 to 2007) 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 . At Manacapuru, the Amazon's water level is only about 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 , in some places as much as . The main river is navigable for large ocean steamers to Manaus, upriver from the mouth. Smaller ocean vessels below 9000 tons and with less than draft can reach as far as Iquitos, Peru, from the sea. Smaller riverboats can reach 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 high and