Water Erosion

Surface Water Erosion

Runoff is water that doesn't soak into the ground or evaporate but instead flows across Earth's surface. Think about when you spill a glass of water and before you can wipe it up it spills into the carpet. That is an example of when rain water doesn't get evaporated or soaked up. Obviously, runoff does not happen randomly, it depends on several factors. For example, if light rain falls for several hours then runoff most likely won't form because the water will have time to evaporate. But, if heavy rain falls for less than an hour the water will not have time to evaporate or soak up and runoff will form. It also depends where the water falls. When it rains on a steep slope, gravity pulls the water down the slope, therefore it becomes runoff. It is runoff because it is roaming the Earth's surface having not been evaporated or soaked up. Now think about it, a roaming mass of water must cause erosion. Runoff mainly causes three types of erosion: rill erosion, gully erosion, and sheet erosion.

Rill erosion occurs when runoff flows and a small stream forms and carries away plants and soil leaving a scar or channel on the side of the slope. A rill is the channel left on the side of the slope.

Gully erosion is when a rill channel becomes much deeper and wider which forms a gully. Gully erosion is basically rill erosion on a much higher level.

Runoff

Gully Erosion

Rill Erosion

Sheet erosion is when water flowing into lower elevations loses energy and leaves behind sediments that cover the soil like a sheet. No one, especially a farmer, is fond of erosion due to runoff. So, farmers put up vegetation (that surrounds their crops) that slows down the runoff, usually stopping erosion.

Sheet Erosion

Surface Water Deposition

Sediments that are carried by water aren't carried very far before they are deposited. Many sediments deposit within the same stream channel. Other sediments travel far distances before they are deposited. Water usually has a lot of energy when it moves down hills or steep slopes. Once it flows on a level surface, the water begins to slow down and lose its energy. The sediments are dropped and deposit into the ground.
Sediments that are deposited as water empties into an ocean, gulf, or lake to form a triangular/ fan-shaped deposit called a delta. When the river water empties from a mountain valley out onto a flat open plain, to form a fan shaped deposit is called an alluvial fan.

By the ThinkQuest Team of Grant School

Photo courtesy of the USGS

Photo courtesy of Rick Mitchell

Photo courtesy of Rick Mitchell

Photo courtesy of Rick Mitchell

Photo courtesy of Rick Mitchell

Photo courtesy of Rick Mitchell