When water enters the floor of a valley, it dumps into a river or stream. Rivers and streams are bodies of water that have a large amount of force. Because of this strength, they can cause a great amount of erosion.
The Grand Canyon in Arizona is a great example of river erosion. Not only was it formed by erosion from moving water, but the water still flows and continues to erode the rock.
The moving water of the river carries away small sediment, leaving pebbles and rocks behind. Because of the movement, the rocks crash into one another. This weathers them into smaller and smaller pieces of rock.
Hydraulic action, abrasion, attrition, and solution are four main things that cause erosion.
Water uses hydraulic action when it dislodges and moves sediment. When the velocity of the water and steepness of the land is greater, the hydraulic action is also greater. Another way the hydraulic action is greater is when the bottom of the water’s surface is rough. The water “grabs” the ridges and lifts them up with hydraulic action.
When a rock’s surface is rough, abrasion smooths the surface back down. It does this using the friction from the constantly moving rock fragments, sediment, and gravel in the water. This is a form of physical weathering.
This process speeds up the more sediment a river (or stream) carries. If there is a lot of sediment, it knocks together and breaks up, causing more weathering to occur. This is because more rocks are hitting the surface more frequently.
Just like in the street on land, the indention’s rocks make on the surface are called potholes. Many potholes happen during flooding. Normally, potholes occur where the rock is softer, or where the flow of water is more narrow.
Attrition is when the surface is eroded by itself. When the surface of the rock is rough, it can sometimes break itself into smaller pieces. This grains in the surface of the rock are broken up, while also making them rounder and smoother. This can even happen with ice in colder areas!
Attrition typically happens upstream, when the velocity of the river is higher. This helps the sediment to be carried faster. When this happens, the surfaces rub against each other more often, and with more force. This increases the speed of erosion.
The speed of the water is responsible for most types of erosion.
Some rocks can be broken down by the slightlyacidic water in a river or stream, which is called solution. Limestone and sedimentary rocks cemented with calcite are susceptible to acidity. What happens is that the calcite dissolves from the rock, which causes it to break down. This is a form of chemical weathering. The pieces of rocks are then picked up by hydraulic action.
For more information on river and stream erosion, watch this video: