Erosion

A River erosion is the gradual removal of rock material from the river banks and bed. As the velocity increases so too does the load it can carry and the rate at which it can erode. As the river winds its way from the source to its mouth it may erode by the following processes:

1. Attrition - The loosened materials that are being carried away collide against the river sides and the bed and against one another. Over time, they would become smaller and eventually reduced to fine particles called silt. This is more likely to occur when the rivers are flowing in the highland areas.



2. Hydraulic Action - The breaking down of rocks and dragging them away from the bed and the banks by the force of the running water itself. When water from a fast moving stream enters cracks in a rock, the force breaks up into pieces. The work of hydraulic action forms plunge pools.



3. Corrasion - The grinding of rock fragments carried against the bed and banks of the river. This action both widens and deepens the channel. The power of the grinding is powerful in floods when large rocks are carried along the river bed. This process acts in two ways (Vertical - depth and Lateral - width) River banks are worn away by a sand papering action known as Abrasion.This is more likely to occur in the lower course of the river by which time materials will have been broken up small enough to be carried in suspension.



4. Corrosion - The solvent action of water dissolves soluble materials and carries them away in a solution. For example, when the river stream flows over an area of limestone, it erodes the limestone by reacting chemically with it and dissolves it. This can occur at any point of the river's course.





Deposition

Deposition is where material carried by the river is dropped. This will occur when:

• There is no longer sufficient energy for the river to transport its material.(Load)
• A River flows across the permeable rocks which allow the water to infiltrate into the underlying rocks.
• When there is a sudden change in the gradient of the river (when the river leaves the mountain and folws onto a plain.
• When the river flows into a lake or a sea.
• When one side of the river is shallower than the other.

As the river flows along the course, it cuts away and erodes the materials from the outsides of the curves in the river. Then the water moves and redeposits the curves in the inside curves of the river. The reason that the outside curves are eroded is because water speeds up as it flows along the outside curve. The water then slows down in the inside curves. This change in speed is what causes the river to deposit its sediments on the inside curves. The constant erosion and deposition causes the channel to meander and changes the river's course overtime. Rivers carry a lot of different sediments, including rocks, boulders, silt, mud, pebbles and stones. Normally, a river has the power to carry sediments. If the force of the river drops, the river cannot carry its sediments. This is when the river deposits its sediments. The largest material will be dropped first as it requires the most energy to be transported. Deposition can be seen in the inside of the meander bend or when the river enters the sea.


Illustration of the deposition of the river



A bit of deposition on the inside of the bend

Here are some great URL's for videos and animations!!!

Erosion:
- http://cgz.e2bn.net/e2bn/leas/c99/schools/cgz/accounts/staff/rchambers/GeoBytes%20GCSE%20Blog%20Resources/Animations/rivererosion_njenkins.swf
- http://www.youtube.com/watch?v=ChEHQUMEkXw
-(Bill Nye the Science Guy episode part 1/3) http://www.youtube.com/watch?v=IR6evrt7els
-(Bill Nye the Science Guy episode part 2/3) http://www.youtube.com/watch?v=vzeB9SacdtA
-(Bill Nye the Science Guy episode part 3/3) http://www.youtube.com/watch?v=HyATTMKxbj4&feature=related

Deposition:
- http://www.classzone.com/books/earth_science/terc/content/visualizations/es0604/es0604page01.cfm?chapter_no=visualization