Rivers are one of the many features present on the landscape of the planet. They are used for a variety of means, such as for water, transportation, sources of food; but for the most part what goes on beneath the surface of the water remains unseen. People don't necessarily notice the processes going on in every river or the land forms that the rivers create over a period of time. There are three main channel processes that occur in every river, and these processes lead to the creation of many different landforms.
The first channel process that occurs in rivers is called transportation. Like the name implies, the river is able to transport materials along the bed or even through the current itself. The size of the materials transported can range from incredibly fine particles like silts, to sands, pebbles, gravel and all the way up to large boulders, depending on the strength of the current and the flow. The smaller materials can be swept up in the current and suspended, while the larger gravels and boulders may roll or even bounce along the river bed.
In areas of the river where the velocity has decreased, the transported particles will naturally slow and may even deposit on the river bed, which is the second channel process. The larger particles will be deposited first and if the velocity slows enough, even the suspended particles could deposit. This process may occur when the river meanders and the velocity slows due to the change in the channel pathway, or perhaps when the river enters a larger body of water and the velocity slows as the larger body is not moving. It may even occur if there is a sudden increase in the amount of transported materials or in the case of flooding, where water overflows onto the floodplain, and the rough surface of the floodplain will slow the water down.
The third and final type of channel process is the opposite of deposition, erosion. Instead of material being deposited, the fast moving water adds particles and materials to the current and it is then transported somewhere else. The erosion of materials could be a result of larger particles impacting the riverbed or breaking up into smaller materials. Turbulent water can also cause riverbanks to dislodge and become part of the loading in the water. Now that the three types of channel processes have been explained, there are several different landforms that these processes can create along a river.
As a river overflows its banks due to a flood, the floodplain is added to. The floodplain is typically a relatively flat area that runs alongside rivers. As water moves onto the floodplain, its velocity is typically slowed by the rough surface of the floodplain, and material such as sands and silts is deposited on the surface of the floodplain. Depending on the composition of the floodplain, material may also be picked up by the flood and the floodplain eroded by the water.
While most people are more familiar with man-made levees in order to hold rising waters back and prevent flooding, rivers can also create natural levees along their banks. These levees are the result of flooding. As the water overflows its banks, sand and silts can be deposited on the edge of the river. Over time, and as a result of more flooding, the material at the edge of the river will build up and form a natural embankment that can help mitigate future flooding.
In some rivers that experience very high short-term flows with high material loading, braiding channels will form along the river's course. The formation of these channels may be the result of seasonal flow, a heavy storm or even snow-melt. The conditions for braiding channels to form begin with a heavy increase in the amount of water coming into the channel, along with a heavy amount of sediment. The velocity suddenly drops, due to the storm ending or the snow-melt being completed, and the loss of velocity results in the heavy deposition of material in the channel. The deposition occurs in different areas and causes the now-lower-flowing water to follow many different channels in the riverbed, which was broken up by large deposits of sediment. The different channels may segment and then converge multiple times along the course of the river, and, until the flow returns, the river will remain braided.
Cutoff and oxbow lake
In meandering rivers, sometimes erosion results in the shortening of the meander. Instead of traveling around the long meandering bend, the water erodes away the land and creates a shorter path in which to flow. At the cutoff, the new river channel will often deposit material in the cutoff bend, resulting in an oxbow lake that becomes completely cut off from the river it was once a part of and creates a separate body of water that is usually crescent-shaped.
As water leaves a river and enters a larger body of water, the slowing velocity will cause deposition and can result in the creation of a river delta. Deltas are so named because they resemble the Greek letter delta. While there are many different types of deltas that can form, the deciding factors have to do with the body of water it is emptying into.
Some deltas form the standard letter shape or a fan shape and are the result of the body of water pushing back on the main tributaries and piling up sediment at the mouth of the river. Other deltas extend out from the land and into the body of water and have few major tributaries but numerous minor tributaries. Sometimes the wave action causes the sediments to pile up evenly on either side of the river mouth, with a single point forming from the single tributary.