The image below shows a river in its middle or lower course. We can notice that the flow of the water is less turbulent compared to what we have seen in the upper course (previous lesson). The river will be flowing along lower-lying areas. Lateral erosion or sideways erosion is more important than vertical (downward) erosion but deposition also occurs. Landforms such as meanders, ox-bow lakes, flood plains, levées and deltas develop.

As the river flows over flatter land it starts to bends. These bends are called meanders (refer to image above). These meanders are not static but over time they change their shape and position as a result of erosion, transportation and deposition. The video below shows clearly how meanders change position depending on the flow of water in the channel.

As we have seen in the video river meanders are the result of both erosion and deposition. As the river flows around the bend, the faster water tends to flow on the outside. This is indicated by the blue arrow in the diagram below. This deepens the channel and the banks of the river are undercut. The water erodes the outside of the river bank to form a river cliff. Deposition, which includes sand and shingle, takes place in the shallow water of the inside of the river bend, forming a small river beach which, with time, builds up to create a gentle slip-off slope. Here the river banks are gently sloping and the river channel is shallow. The first image shows a series of meanders. Notice the deposition on the inside of the meanders (pale material) and the river cliff (indicated by dark shadows) on the outside of the meanders.

The diagram below shows a cross section along a river cliff. Note that there is deposition in the inner bend where the current is slow and erosion on the outer bend where there is a faster moving current. The deposited material creates a slip-off slope while a river cliff is created on the outside bend.

As the erosion and deposition continue, the curve becomes larger and more circular and the neck of the meander becomes narrower. Eventually the river cuts right through the neck and shortens its course through a new more efficient path. Sediment is then deposited that blocks off the old bend cutting off the loop from the stream entirely. This results in a horseshoe-shaped lake that looks exactly like an abandoned river meander. Such lakes are called ox-bow lakes because they look like the bow part of the yoke formerly used with teams of oxen. The diagrams below show in stages how an ox-ow lake forms.

Ox-bow lakes are still lakes, generally, no water flows in or out of oxbow lakes. They rely on local rainfall and, over time, can turn into swamps. Often, they ultimately evaporate in just a few years after having been cut off from the main river.

The following video explains clearly how meanders and ox-ow lakes are formed.

The diagram below shows a wide floodplain. This consists of a wide flat area of land over which meanders are found. A floodplain is the area of land that will be covered with water when the river overflows its banks and floods. Each time the river floods, a thin layer of material called silt is spread over the floodplain. As a result a floodplain is a very fertile area and good for farming. This fine fertile material is known as alluvium.

Every time the river floods the largest sediment is deposited close to the channel as the river starts to loose energy. Smaller sediment is deposited further away as a smaller amount of energy is required to transport it. The deposited material forms natural embankments called natural levées. Sometimes levées are strengthened to protect the surrounding area from flooding. The three diagrams below show how natural levées are formed.

The video below explains the main characteristics of floodplains in the middle and lower course of rivers, and how the processes of sediment transport and deposition (during flood events) lead to the formation of levées.

https://timeforgeography.co.uk/videos_list/rivers/floodplains/

Some large rivers like the Amazon, Nile, Niger, Brahmaputra and the Mississippi transport huge amounts of sediment down to their mouths where they meet the sea. The material carried by the river finds itself in the sea and gradually builds up new land called a delta. This is the place where the river deposits the material and sediment which it will have carried. These are low and flat lands and therefore are in danger of being submerged if the sea-level rises. However, these deltas provide fertile and humid land which are easily farmed. This is precisely the reason why these deltas attract thousands of people who make a living there. In fact the Nile delta in Egypt and the Ganges delta in Bangladesh are two of the most densely populated lands in the world.

A delta is formed when a river deposits its material faster than the sea can remove it. Two main types of deltas are Arcuate and Bird’s Foot.

Arcuate or fan-shaped deltas have a triangular shape like that of River Nile. The land around the river mouth arches out into the sea and the river splits many times (distributaries) on the way to the sea, creating a fan affect. The Nile Delta (shown in green in the map below) flows out into the Mediterranean Sea. The Mediterranean has no tides and so does not have enough energy to remove the silt deposited by the river.

Bird’s foot deltas like the Mississippi delta has a shape of a bird’s foot. The material deposited builds new land outwards in the sea. The river splits on the way to the sea each part of the river juts into the sea. In the satellite picture below we can see the delta of the Mississippi river in the Gulf of Mexico.

These two videos explain how levees and deltas are formed.

Now work out the worksheet entitled River Landforms in Middle and Lowland Areas Worksheet available in the resources section of this lesson.