The Amazon watershed includes the largest and wettest tropical plain in the world. Heavy rains drench much of the densely forested lowland region throughout the year but especially between January and June. The width, rate of flow, and discharge volume of the river reflect seasonal variation in rainfall. During the months of maximum precipitation, broad areas traversed by the Amazon are subject to severe floods. In Brazil the width of the river ranges between 1.6 and 10 km (1 and 6 miles) at low stage but expands to 48 km (30 miles) or more during the annual floods; the rate of flow ranges between 1.5 and 5 mph; and the crest of the water at flood time often rises 15 m above the normal level. To drain the vast mass of water, the Amazon has carved a deep bed in the plain through which it flows. In one sector near Óbidos, Brazil, the bed is more than 91 m below the average surface level of its water.
Because of its vastness, annual floods, and navigability, the Amazon is often called the Ocean River. The total number of its tributaries is as yet uncounted, but more than 200 are in Brazil alone. Seventeen of the largest known tributaries are more than 1600 km in length. The Amazon proper is navigable to ocean liners of virtually any tonnage for two-thirds of its course. Transatlantic ships call regularly at Manaus, nearly 1600 km upstream; and smaller ships can reach Iquitos, Peru, 3700 km from the river's mouth, the farthest point from sea of any port serving ocean traffic. River steamers of more modest tonnage can navigate on more than 100 of the larger tributaries.
The Upper Course
This is the first part of the river and begins at the source. Rivers flow through V-shaped valleys in their upper course. V-shaped valleys are usually found in the mountains and hills. They are so called because they often have very steep sides.
V-shaped valleys are formed by erosion. The river carries stones and rocks in its water. The force of the water and the grinding of rocks and stones cut down into the river-bed to carve out a valley. Over time the valley becomes deeper and wider:
A gorge is a steep-sided river valley, which is very narrow and deep. Most gorges have rocky sides. The river cuts this deep valley by erosion. Gorges are created over thousands of years.
A waterfall is formed where water flows over a cliff or very steep drop in the river's bed. Waterfalls develop in several ways. Different rates of erosion where a resistant layer of rock in a streambed overlies softer layers is the principal manner. Subsequent erosion of the softer rock by the falling water undermines and periodically breaks off portions of the harder cap rock. Some of the largest cataracts in the world, Niagara Falls in North America and Victoria Falls in Africa, for example, originated in this way.
Waterfalls are usually found in the upper course of a river in the hills or mountains. The Dunn's River Falls in Jamaica is the only known waterfall which forms the mouth of a river as it enters the sea.
With interlocking spurs, the volume of water is small and energy is used to overcome friction therefore, the river has little energy to erode the banks. The river thus has to flow past any barrier in its path such as spurs. In the process of doing so, it develop a winding course. As the river past alternate spurs on both its sides, it appears to interlock.
The Middle Course
A meander is a bend in a river. Meanders normally occur in the middle and lower courses where the water is moving more slowly. The river carves out S-shaped bends. A meander belt is where there are several meanders occurring one after the other.
Meanders are formed by erosion. As rivers move from source to mouth, they carry sediment and other material. Energy is needed to carry this sediment. But if the river has some spare energy, it can erode or wear away its bed, banks and other parts of the river.
Most erosion normally occurs on the outside bend of a meander. This is because the water has further to travel. Also, pieces of sediment may be thrown against the riverbanks to wear them away. Evidence of erosion on a riverbank changes from river to river. Sometimes you can see tree roots hanging. Often there is a steep side to the outside riverbank. The bank may overhang. There may be pieces of sediment in the river. Where a riverbank overhangs, it is possible to put a hand or foot underneath the hanging part.
When meander bends become giant loops, there is a thin piece of land left between the beginning and the end of the meander. This is the meander neck.
As the river neck becomes very narrow, the river can break through. For a short time, water flows both round the meander (which is now called a backwater) and across the meander neck.
Eventually the river cuts off the backwater completely and flows across what used to be the meander neck. For a short time, an oxbow lake is left behind. It is called an oxbow lake because it is shaped like the old fashioned 'U' shaped yoke that was once used to hitch an ox to a plough. The oxbow lake lasts until it becomes overgrown with weeds and filled in with soil. This happens quite quickly as it is cut off from the main river and therefore doesn't get any water.
The Lower Course
The floodplain is the flat land of the river valley close to the riverbanks. The floodplain is usually found in the lower course of a river. It is a fertile area of land, used for agriculture and growing crops. The floodplain is covered with water when a river floods. Alluvium is a type of mud that is left behind when the floodwaters go. Alluvium is very fertile.
When river discharge exceeds the capacity of the channel, water rises over the channel banks and floods the adjacent low-lying lands. As water spills out of the channel some alluvium will be deposited on the banks to form levees (raised river banks). This water will slowly seep into the flood plain, depositing a new layer of rich fertile alluvium as it does so.
A levee is a naturally formed raised bank along the side of a river channel. When a river overflows its banks, the rate of flow is less than that in the channel, and silt is deposited on the banks. With each successive flood the levee increases in size so that eventually the river may be above the surface of the surrounding flood plain. Notable levees are found on the lower reaches of the Mississippi in the USA and the Po in Italy.
Rivers move slowly at the river mouth. The river carries a lot of sediment and mud. The river deposits its sediment and mud at its mouth. This sediment and mud spreads out into a fan- like shape, or delta, across the mouth of the river.
If the force of the sea tides and water is not very strong, the mud deposits cannot be washed away. Instead, the deposits break up the flow of the river water into many smaller channels. The delta gradually gets bigger and bigger. The new land is made up of billions of pieces of mud, sediment and small rocks. This sediment has been carried by the river from its source to its mouth.
A delta is often triangular in shape. It gets its name from the Greek letter 'delta' which is shaped like a triangle. Not all deltas look the same, but they all split the river into many different channels. The channels in a delta are called distributaries.