Typically, a river valley can be divided into three sections; the upper course, the middle course and the lower course. There are particular river features which we can expect to find in each of these sections. We shall examine each section of the river along with their various features in some detail. The diagram below shows the three sections of a river.
River Features found in the Upper Course
This is the part of the river closest to the source. The gradient is steep. The river is small and flows quickly. Some features expected in this section of the river are V- shaped valleys, interlocking spurs, a river bed with large rocks and stones,gorges, fast flowing rapids and waterfalls.
The river flows quickly cutting down into its bed. This process is is known as vertical erosion. It forms deep valleys with steep sides know as V-shaped valleys. Take a look at the V-shaped valley in the picture below.
As the river meanders around obstacles, erosion is concentrated on the outside of the bends. At the same time, vertical erosion is taking place. The result is ridges of high land which project toward the river decreasing in height toward the river. These ridges alternate on either side of the river and are called interlocking spurs. Click here for a diagram showing interlocking spurs.
When a river plunges vertically over a rock face, the result is a waterfall. Waterfalls are formed in several ways, for example, a river may plunge over the edge of a plateau. It is common for waterfalls to form where a river crosses a band of hard rock or resistant rock. This is shown in the video below.
As a waterfall retreats upstream, it leaves behind a narrow steep sided feature known as a gorge. Gorges can also be formed in other ways. The Grand Canyon of the Colorado River in the USA formed as the surrounding land was uplifted. Take a look at the photos below.
Watch this video about how gorges can be created by waterfalls.
At rapids, a river flows very fast down a steep slope. The river bed is rough and rocks are exposed at the surface. There may be alternating bands of hard and soft rocks which cause an uneven river bed. These conditions create a zone of turbulent water called rapids. The picture below shows Scales Rapids in the Gauley River in the USA.
River Features found in the Middle Course
In this part of the river, the gradient is not as steep as it was in the upper course. Lateral erosion (erosion of the banks) is now more effective than vertical erosion. The river develops a wider, more open appearance. Some deposition now takes place. Meanders (bends in the river) are more common. Typical features found in the middle course are:
River cliffs and slip-off slopes:
As a river meanders, the outside banks of the meander are undercut. This creates river cliffs. On the inside of the meander, where the water flows more slowly, deposition takes place. This gives rise to a gentle slope known as a slip-off slope.
River Features found in the Lower Course
This is the part of the river nearest the sea. Here, the gradient is even gentler. The river flows even more slowly and the volume of water is greater. Deposition is much more important than erosion in this part of the river. Some features usually found in this section are:
In the middle and lower courses of a river, alluvium is deposited on the valley floor, mainly where the river floods. The wide flat deposits which result are known as the flood plain of the river. Flood plains vary from a few hundred meters to several kilometers across.
These are shallow crescent shaped lakes formed when a meander is abandoned by a river. Sometimes a meander becomes so pronounced that only a narrow neck of land separates the two ends of it.When the river is in flood, this neck is broken and the river starts to flow a shorter, more direct route. Deposition takes place, sealing off the ends of the meander and creating an ox-bow lake. Take a look at the animation below which shows the formation of an ox bow lake.
This photo, taken near The Last Frontier Boutique Resort is courtesy of TripAdvisor
When a river is in flood, it deposits material on its flood plain. The heaviest material is deposited near the river. After successive floods, natural embankments called levees are created alongside the river. This is shown in the diagram below.
This occurs when a river divides for various distances into two or more channels. The channels are separated by islands of sediment called bars. Braiding occurs when the river carries a large load or when the volume of the water in the river changes rapidly from season to season.
Much of the material carried by a river ends up in the sea. If the conditions are favorable, this sediment may accumulate until it rises above the surface of the sea. This area of flat land, built up from silt and alluvium where a river flows into the sea is called a delta.The build-up of sediment eventually interferes with the flow of the river, causing it to split up into several channels called distributaries.
Relatively few rivers have deltas. Deltas form most easily under the following conditions:
- when the river carries a large volume of sediment
- when the sea is relatively calm,so the sediment is not carried away by waves or strong currents.
- where the sea is not too deep
- when the river flows slowly
- when the vertical difference between high tide and low tide (tidal range) is not too great.
The Nile Delta in Egypt is a piece of land that was built as the Nile dumped sediment into the Mediterranean Sea. Take a look at the photo of the Nile Delta and the diagram below it.