Estuary: the Amazon Meets the Atlantic
The enormous volume of freshwater that flows through the Amazon River discharges into the ocean in the place known as the Estuary. There, saltwater gives way to freshwater, though during the low-water season salt water invades parts of its southern side. The estuary is characterized by a series of islands that form archipelagos, with Marajó Island the largest. Flowing through the islands, the Amazon River channel discharges to the north of Marajó, but some of its waters flow to the south through the Breves strait.
Unlike what occurs in other parts of the Basin, the change in water levels in the estuary is only about 0.5 m during the high-water period, because the level of the river is controlled by the daily tides of the Atlantic, whose maximum tidal levels are approximately 4 m. The tidal influence also reaches upstream near the mouth of the Xingu River, 200 km from the Atlantic, where the maximum tidal range is approximately 1.9 m. During low water, the average tide height is 1.3 m. The mouth region of the Amazon River is relatively shallow with depths usually less than 10 m. Channel depths average approximately 20 m near Macapá, the capital of the state of Amapá.
Given the enormous volume of water discharged through the island channels, the river waters discharge into the Atlantic Ocean, and are deflected northward by the Southern Equatorial Current. Thus, the water along the coast of Amapá is brackish, though often with a freshwater layer on top. Amazon River discharge prevents the invasion of saltwater into its mouth area. Evidence of this is the vast tidal forests made up almost entirely of rainforest species.
The Amazon River produces a strong tidal bore known locally as the pororoca, a solitary wave that is more evident during full moon and new moon. The phenomenon originates when shallow areas of the river, usually shallower than 4 m, are overcome by the force of incoming tides. The bore is noticeable only during these tidal surges, when it can reach 2 m in height and travel 10–15 km per hour upstream. The bore’s force is sufficient to erode riverbanks and cause huge trees to crash into the water. Local mariners consider the Amazon River pororoca to be dangerous.
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Mangrove along the Amapá coast just north of the Amazon River mouth that indicates a mixture of fresh and salt water. Department – Country: Amapá – Brazil Main Basin – Sub Basin: North Coast Basins – Araguari Elevation: 14 Photographer: Michael Goulding
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Enormous mudflat near the mouth of the Amazon River built with sandy sediments transported mostly from the Andes more than 4,000 km upstream. Department – Country: Pará – Brazil Main Basin – Sub Basin: Estuary – South Amazon Estuary Elevation: 3 Photographer: Michael Goulding
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Maximum tidal levels are indicated by the naked stems near the fish trap. Department – Country: Pará – Brazil Main Basin – Sub Basin: Estuary – South Amazon Estuary Elevation: 10 Photographer: Michael Goulding
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Low tide near the town of Vigia where brackish water occurs during part of the low water period of the Amazon River. Department – Country: Pará – Brazil Main Basin – Sub Basin: Estuary – South Amazon Estuary Elevation: 10 Photographer: Michael Goulding
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The estuary near where Marajó Bay meets the Atlantic. During the Amazon River’s high water period freshwater extends out of Marajó Bay and along the Atlantic coast. Department – Country: Pará – Brazil Main Basin – Sub Basin: Estuary – South Amazon Estuary Elevation: 19 Photographer: Michael Goulding
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Tidal floodplain of Marajó Bay, about 100 km inland from the Atlantic. Department – Country: Pará – Brazil Main Basin – Sub Basin: Estuary – South Amazon Estuary Elevation: 10 Photographer: Michael Goulding