Composite satellite photograph of the Amazone estuary (source: Wikipedia)
The Amazon basin is the largest hydrographic system in the world. The river in itself length between 6,259 km and 6,800 km and so compete with the Nile (which length is between 6,499 km and 6,700 km) to be the longest river in the world. Generally speaking, the Amazon river is some kind of a record breaker! Such a complexity poses problem in collecting data, and so those I present here are nothing but a compilation of studies I know and the state of my work. These can be questioned sometime, even by me.
The average transport of the river is about 170,000 m3 · s-1, with minimum and maximum values at Óbidos (700 km upstream of the river mouth) of 69,000 m3 · s-1 in 1906 and 282,000 m3 · s-1 in 1989 (Callède et al., 2002). A recent survey indicates that the average flow at the mouth is of the order of 200,000 m3 · s-1 (Callède et al., in press). It also transports a large amount of particles and sediments: Milliman and Meade (1983) estimate that (1.2 ± 0.1) · 109 tons of suspended particles and 2.3 · 108 tons of dissolved material are transported yearly into the Atlantic ocean by the Amazon river. Its lower course is very flat: 20 m of variation in level over the last 1,500 km. As a consequence, the river flow is mostly constrained by upper water discharge and tides. The flow is so strong that seawater never enters the estuary, resulting in a large pool of brackish water beyond the mouth (e.g. Callède et al., in press; Gordeev et al., 1992). This pool oscillates with the tide and, after leaving it, waters from the river extend in a plume over hundreds of kilometers into the Atlantic Ocean along the South American coast under the influence of the North Brazil Current and the trade winds.
According to Nikiema et al. (2007), local currents, mainly the North Brazil Current, play a key role in determining the fate of the Amazon Rivier discharge. The North Brazil Current is fed by the South Equatorial Current, and runs northwestwards along the Brazilian coast with a transport of about 35 Sv (1 Sv = 106 m3 · s-1) and over 300 km width near 44° W, with a seasonal variation of about 3 Sv. The circulation on the shelf is mainly forced by trade winds. Northeast trade winds dominate from December to May, having high monthly mean intensities that can reach 18 m · s-1 in February. Southeast trade winds blow during summer and autumn with lower intensities (less than 12 m · s-1).
The shape of the Amazon estuary is extremely complex, as many of channels and mangroves make the separation between dry and wet land difficult to define. Furthermore, the Amazon flow causes very fast erosion and sedimentation processes, so that the river bed changes rapidly with time, and one can observe variations from one season to another. Part of the Amazon discharge goes through the numerous channels down to the Pará river. Though it represents less than 5 % of the flow of the Amazon River, about 30 % of waters in the Pará comes from the Amazon (Callède et al., in press). Usually, the Amazon River mouth itself is considered to extend from Cabo do Norte to Punto Patijoca, about 330 km wide.
The river is connected to a large shelf extending from Venezuela (10° N, 60° W) to the eastern edge of Brazil (5° S, 35° W). Gallo and Vinzón (2005) indicate that the estuary is classified as macro-tidal, with a tidal range between 4 m and 6 m, and as semi-diurnal, with a wave shape relationship (the quotient of the diurnal to semi-diurnal harmonics) equal to 0.1. This shelf is very flat and shallow. For instance, according to the bathymetry published by Gallo and Vinzón (2005), the 20 m isobathymetric line can be more than 100 km away from the Amazon mouth.
According to Kosuth et al. (1999), during low water, tidal effects can be measured 1,000 km upstream. During high water, because of the greater flow speed, the tidal wave is damped. Nevertheless, tides propagate to Santarém, almost 800 km upstream.
I wish to thank Jaques Callède for giving me important precisions, which helps me to correct the information I am giving here. This page also owe so much to Leonardo Dardengo, who died in the crash of Air France flight AF447, on June 1st, 2009.
