The mouth of the Amazon River

Located in the tropical rainforest area, the precipitation is more than 2,000 mm, with dry and wet seasons, and most of the rain comes from the wet season. The maximum flow is 225,000 m3/s; the minimum flow is 90,000 m3/s, which occurs in May-June and November-December respectively. The average flow is 175,000 m3/s, which is equivalent to 4 times that of the Congo River and 10 times that of the Mississippi River. The Amazon River discharges 693 billion cubic meters of water into the sea every year, accounting for one-fifth of the total runoff into the sea in the world. If it is spread into a 10-meter-thick water layer, its area is equivalent to the North Sea (575,000 square kilometers).

The average annual sediment transport is 1 billion tons, mainly silt and clay. There are obvious seasonal changes in the sand content. The average maximum sand content in each month of the wet season is 125 mg/L; while in the dry season it is 22 mg/L. Coarse materials are deposited in the lower reaches of the river, forming a series of sand islands. Among them, Marajo Island between the Amazon River estuary and the Para River estuary is the largest. It is 290 kilometers long and 161 kilometers wide. The Amazon River estuary is divided into the North Trough and the South Trough and the Para River estuary by sand islands, which become three main channels into the sea. 95% of the output sediment enters the sea in the form of suspension.

The tide is a semi-diurnal tide type, with a maximum tidal range of about 11.8 meters near Maraca Island, and then gradually decreases both toward the sea and upstream. The tidal velocity is 2.5 meters/second. Due to the wide mouth, the narrow river upwards, and the large resistance of the numerous sand islands in the river channel, the tidal bore phenomenon (locally called Pororoco) occurs, with a tidal height of about 1 to 2 meters, which can reach about 5 meters during low water. Its momentum is very spectacular, and its sound can be heard 1 to 2 kilometers offshore. If the flood tide meets strong winds, the water level will rise sharply, up to 15 to 30 meters, causing serious flooding in the lowlands on both sides.

After the flood season diluted water (salinity less than 10) flows out of the estuary, a 100-180 km muddy water zone is formed outside the estuary; along the coast, a turbid belt with an average width of about 60 kilometers is formed, extending 2,000 kilometers to the northwest. The sediments carried by the river water participated in the shaping of the beach profiles along the coast of Suriname and Guyana and the Orinoco River Delta. The waters outside the estuary can be divided into three zones: that is, in the range of salinity 0 to 10, there are many terrigenous materials, the water is brown, and it is the terrigenous material zone; the waters with a salinity of 10 to 15 contain many diatoms and are green, which is the biological material zone; the waters with a salinity greater than 25 are blue, which is the blue seawater zone.

The flow system on the coast outside the estuary is very complex. The Guyana Current is strong, with a flow rate between 0.25 and 2.00 m/s.

In March and April, the North Brazilian Coastal Current, a branch of the South Equatorial Current, merges with the Guyana Current at a flow rate of 1.10 m/s; there is also a weaker coastal current to the south. The salinity stratification phenomenon is significant, on the inner shelf 60 to 185 kilometers outside the mouth in the dry season and within 80 to 230 kilometers outside the mouth in the wet season.

On the inner shelf within the 40-meter isobath outside the mouth, the sediments are mainly fine-grained materials, which is exactly the opposite of the classic sedimentary model on the continental shelf, that is, the inner shelf is coarse-grained materials, the middle shelf is fine-grained materials, and the outer shelf is residual sediments. This is due to:

① The fine-grained sediments of the Amazon River (mainly silt and clay) and