The Amazon Estuary is a stretch of the Amazon River that extends more than 190 miles (300 km) inland from the river’s mouth to an area southwest of the city of MacapÃ . Varying in width from 15 to 190 miles (25 to 300 km), the estuary is partly filled by numerous low-lying, forested islands.
The Amazon Estuary has by far the largest water output of any estuary in the world, discharging an average of 46 million gallons (175 million liters) per second into the Atlantic. The sheer magnitude of this discharge means that, almost uniquely among estuaries, there is very little saltwater intrusion into it. Instead, nearly all of the mixing between the river’s discharge and seawater occurs outside the estuary, on an area of continental shelf. Despite the relative lack of seawater intrusion, the whole Amazon Estuary is significantly affected by twice-daily tides, which cause inundation (by river water) of most of the islands in the estuary.
The Amazon Estuary's Pororoca Surf
Tidal bores, locally called pororocas, occur on large spring tides in several of northern Brazil’s river estuaries. Some of these bores attain heights of 10 ft (3 m) and can be surfed for several miles. This sport is rather hazardous, however, because the waters through which the pororocas surge are home to dangerous snakes, fish, and crocodiles.