Water Erosion Definition
Water erosion is the removal and transportation of soil, sediment, or mineral components of a rock by a moving, liquid water source. This source could be a river, stream, coastline, rainfall, flood, channel, etc. Water erosion can occur at very low rates that may displace materials just a few centimeters, or at high rates that can cause a total displacement of many kilometers. The amount of liquid water acting as an erosive agent and the steepness of the surface the material is resting on are the factors that will determine the rate of erosion. The amount of water present determines the mass of the erosive agent, while the slope of the surface will determine how much velocity can be generated. Therefore, these two factors combined create the conditions that determine how much force the water will generate. As the amount of water involved in an erosive event increases, so will the size of the particles the water is able to move. Water erosion may involve the transportation of only sand size or smaller grains, or may displace large boulders or entire sheets of rock at one time.
Types of Water Erosion
Whenever and wherever it rains on land, water hits the Earths surface and immediately moves to the lowest place it can be. This process is known as surface runoff, and is a type of erosion that is prevalent everywhere on the Earths surface. One way surface runoff can transport soil and sediments is called sheet erosion, where the amount of rainfall is significant enough to move materials downward with the flow of the rainwater. If the amount of rainfall increases, water will start to move faster towards the lowest point it can reach, favoring certain paths over others. As more water rushes down the same path, the path will deepen and become a channel that will be the preferred route for rainwater to take. This is referred to as gully erosion, which looks like an impromptu stream after heavy rainfall.
Rivers and Streams
River and stream formation is similar to the process of gully erosion described above. This difference is that a river or stream has a consistent source that allows for water to move through the channel constantly (or at least seasonally). Because a river or stream is always running, it can make dramatic changes to its surroundings over the course of its lifetime. Probably the most dramatic example of erosion caused by a river is the Grand Canyon of Arizona, with the Colorado River running through it. However, wild rivers are able to cause much more erosion than just deep V-shaped canyons. They can meander to create dramatic loop shapes, only to cut off the loop and leave a lake there later on. Wild rivers flood frequently and cause erosion not only to the river’s path but the entire floodplain around it.
Materials that are eroded away by a river or stream end up suspended it the flow in parts of rivers that are turbulent, while in more calm waters sediment will settle out of the water and be deposited in the stream bed. Eventually, most eroded materials will end up in the body of water the channels leads to, often in a sediment deposit called a delta. However, in rivers that are dammed, we see that much sediment gets stuck above the dam at the bottom of a man-made lake called a reservoir. This causes both engineering and environmental issues as the reservoir fills up with sediment providing less room for the water that the reservoir was intended to hold.
Coastal erosion is constantly changing the way rock and sediment is deposited on the coastlines, determining the shape of entire continents. Coastal erosion is usually associated with the removal of sediments from the coast, causing the coastline to steadily retreat over time. However, coastal currents play a role in redistributing sediment along the coast so that not all of the eroded sediment is being washed out to sea. Depending on the geologic nature of the coastline, coastal erosion can produce a variety of rock formations, ranging from tunnels and natural bridges to steep cliffs.
Flooding occurs when the amount of rainfall oversaturates the underlying material of an area (soil, bedrock, asphalt, etc.), causing water to build upon the surface. Floods can result in dramatic events of erosion, particularly in cities where much of the ground is covered in relatively non-porous concrete. Due to the high volumes of water present, flood conditions generate enough power to displace sediments, from the size of a grain of sand to boulders the size of cars, a great distance.
Examples of Water Erosion
In some cases, flooding is not the result of rainfall, but rather the result of the release of a great amount of water, such as from behind a dam. One such case of this are the Missoula Floods; a series of catastrophic floods that took place at the end of the Last Glacial Maximum. The source of the water in the case of these floods was Glacial Lake Missoula, which formed in the wake of the retreating ice sheet, and filled with water as it melted. The lake was dammed by ice, and as it melted it released massive quantities of water at a time. This is an event known as a glacial lake outburst flood. A flood of this size causes enormous amounts of erosion, and today we see evidence of the Missoula Floods across eastern Washington and the Willamette Valley in Oregon.
Follow this link for photos of Dry Falls, the site of the largest waterfall that ever existed and the canyon below, both formed via erosion by the powerful outburst of water in the Missoula Floods.
A much less dramatic but very common example of water erosion is the displacement of agricultural soil by surface runoff; an issue that farmers have faced since the invention of agriculture. Surface runoff is a problem on farms because it carries away topsoil that is vital for the success of crops. The topsoil can end up in waterways, carrying herbicides, pesticides, and other pollutants with it, therefore damaging aquatic ecosystems. To mitigate the effects of surface runoff, people in the American southwest have been using acequias for hundreds of years. An acequia is a manmade irrigation ditch that can help to transport water to farms that are not near to a water source but also directs snowmelt and rainwater to a channel creating gully erosion, which does damage to a smaller area than sheet erosion.