13/9/07The Middle and Lower Course of a River

What are the main processes that operate in the middle and lower course of a river?Erosion is still an important process. The river is now flowing over flatter land and so the dominant direction of erosion is lateral.

The river has a greater discharge and so has more energy to transport material. Material that is transported by a river is called its load.

Deposition is also an important process and occurs when the velocity of the river decreases or if the discharge falls due to a dry spell of weather.

Middle and lower courses of rivers have a higher discharge than the upper course because water from the rest of the drainage basin has drained into the river in its middle and lower course.

How is material transported downstream?- Traction This is where boulders and pebbles are rolled along the

river bed at times of high discharge.- Saltation Sand sized particles are bounced along the river bed by

the flow of water- Suspension Fine clay and sand particles are carried along within

the water, even at low discharges- Solution Some minerals dissolve in water such as calcium

carbonate. This requires very little energy.

What landforms are found in the middle and lower course of a river?Landforms found can include:

1. Meanders, created by deposition and erosion2. Oxbow lakes, created by deposition and erosion3. Floodplains and Leveés, created by deposition4. Deltas, created by deposition

How are these landforms created?MeanderA meander is a bend in a river. On the outside of a meander bend you get a river cliff. On the inside you get a slip-off slope.

As the course of a river approaches its middle stages it flows over flatter land. Lateral erosion dominates as the river swings in large bends known as meanders. Meanders constantly change their shape and position.

Water is pushed to the outer bend. This reduces friction with the bed and banks. So the river has more energy for transporting material which can erode the outside bank via abrasion.

Oxbow LakesHow is an oxbow lake formed?

1. In a meander the water is pushed to the outside bend.2. Greater velocity means that the river ahs more energy to erode3. Processes such as corrasion will cause lateral erosion4. Continual erosion on the outside bend narrows the meander

neck.5. The river floods and takes a shortest route, cutting through the

neck. 6. The fastest current is now in the centre of the channel.7. Deposition occurs along the banks of the river.8. The meander becomes cut off to leave an oxbow lake.9. The lake will slowly dry up unless rainfall is very high.

FloodplainsFloodplain FormationFloodplains and levees are formed by deposition in times of river flood. The river’s load is composed of different sized particles. When a river floods it deposits the heaviest of these particles first. The larger particles, often pebble-sized, form the levees. The sands, silts and clays are similarly sorted with the sands being deposited next, then the silts and finally the lightest clays. Every time the river floods deposition builds up the floodplain.

DeltasDelta Formation

Deltas are found at the mouth of a river, where the river is carrying too much load for its velocity and so deposition occurs. The top of the delta is a fairly flat surface. This is where the coarsest river load is dropped. The finer particles are carried into deeper water. The silt is dropped to form a steep slope on the edge of the delta while the clay stays in suspension until it reaches the deeper water.

A delta is usually composed of fine sediment which is deposited when a river loses energy and competence as it flows into an area of slow-moving water such as a lake or the sea. When rivers like the Mississippi or the Nile reach the sea, the meeting of fresh and salt water produces an electric charge which causes clay particles to coagulate and to settle on the seabed, a process called flocculation.

Deltas are so called because it was thought that their shape resembled that of delta, the fourth letter of the Greek alphabet (Δ)/ in fact, deltas vary greatly in shape but geomorphologist’s have grouped them into three basic forms:

- arcuate: having a rounded, convex outer margin, e.g. the Nile- Cuspate: where the material brought down by a river is spread

out evenly on either side of its channel, e.g. The Tiber.- Bird’s foot: where the river has many tributaries bounded by

sediment and which extend out to sea like the claws of a bird’s foot, e.g. the Mississippi.

Although deltas provide some of the world’s most fertile land, their flatness makes them high flood-risk areas, while the shallow and frequently changing river channels hinder navigation.

Key Ideas- Processes of erosion, transport and deposition operate to create

the landforms of the middle and lower course of a river- There are four types of transport: traction, saltation, suspension

and solution- The main direction of erosion is lateral- Erosion and deposition contribute to the formation of meanders

and ox-bow lakes- Deposition is the main process contributing to the formation of

floodplains, leveés and deltas.

Hjulstrom graph shows the relationship between velocity and particle size being transported. It basically shows that the higher the river discharge the higher the potential to carry larger parts of load. The bottom part of the graph shows with what discharge levels the sizes of particles would not be eroded. The erosion velocity is the velocity at which the particles would be eroded or transported in suspension. The

middle part of the graph shows just where the particles would be transported, but possibly not in suspension.