Rivers carry astonishing amounts of water, but where does all that water come from? Do you think that it probably comes from the rain? But then why is there water in the rivers even when it is not raining? In reality, even when it rains, there are only relatively few drops that fall in rivers. The most rain falls directly into the oceans, and the rest mainly falls on the ground or vegetation.
Moreover, part of the rain evaporates and returns to the atmosphere very quickly, but also ends up evaporating quite quickly. There is a small amount of rain that remains. Especially when it rains a lot, some of the water will run off the surface, slide down the slopes and finally collect in rivers.
How do river form?
A river is a natural flowing watercourse, usually freshwater flowing towards an ocean, sea, lake, or another river. River forms from water flowing from a higher elevation to a lower elevation due to gravity. Most rivers originate high up in the mountains. The place where a river originates is called the source of the river. When glaciers melt due to the sun’s heat, it turns to water and flows down the mountain in small streams, gradually forming a river.
Rain also plays a role in the formation of rivers. When rain falls on the land, it either seeps into the ground or becomes runoff, flowing downhill into the rivers. In comparison, rivers are trying to find the quickest way towards the sea. Due to the lot of pressure and force that the fast-flowing river gives, it tends to erode the riverbank and soil and take it with it.
Rivers form snake-like patterns as they flow across the fairly flat valley floors. The fast-flowing side of the river erodes comparatively more than the slow side of the river. Therefore more sediments get deposited on the slow side resulting inland in one side. And on the other side, water-creating loops are known as meanders.
All the rivers and streams start at some high point. These streams slowly join together to form larger streams. Eventually, all this water forms rivers and runs into the ocean. The place where a river enters the sea, ocean, or lake is called its mouth.
From the river’s source to its mouth, the forces of moving water erode and shape the land. We call the action of flowing water on fluvial land processes. Fluvial processes can be broken down into three main components of erosion, transportation, and deposition.
For example, a river such as the Yangtze begins its 6300-kilometer journey as meltwater from the glaciers on the Qinghai-Tibet plateau, where it erodes mountain sediment. As a river winds its way eastward across China, it erodes and changes the landscape transporting more material. Eventually, the river reaches the sea, where the material is deposited in the Yangtze River Delta.
Moving water in the form of rivers is an agent of erosion. River water wears away rocks and soil found on the riverbed and banks. Rock particles being carried downstream continue to break down further as the river transports them. There are four main forms of fluvial erosion.
Hydraulic action: The force of river water against the banks can cause air to be trapped and pressured in cracks in the rocks on the riverbank. This continual pressure causes the rocks to crack further and eventually break away.
Abrasion: As the river transports rocks downstream, these rocks wear down the material on the riverbed.
Attrition: Attrition is where the rocks and pebbles being carried by the river smash against each other, Thereby breaking each other down into smaller, rounder, smoother pebbles.
Solution: Solution or corrosion is the chemical erosion of the rocks of the riverbank. It is because water is slightly acidic, especially where streams flow through rocks like limestone.
Once it has been eroded, material in the river is transported downstream. It depends on the interaction between the velocity of a river and the size of the particles. As a rule, the larger the particle size, the higher the velocity of moving water is required to transport it. When the river’s course is steep, often near its source, velocities are higher, enabling the transportation of large rocks and boulders. During times of flood, energy levels will be even higher.
Transportation takes place in four ways:
Solution: This is where the smallest particles of minerals are dissolved in the water. This usually happens in the middle and lower reaches of a river as it takes some time for the material to be dissolved.
Suspension: Small particles are carried along in the river. This can take place anywhere along the course of the river.
Saltation: Small pebbles bounce along the riverbed. This mainly takes place in the upper and middle sections of the river.
Traction: This is where large boulders and rocks are rolled along the riverbed.
This mainly occurs in the river’s upper reaches as it is the only place where energy is high enough to do so.
When a river loses energy, it will deposit some of the load it is carrying. Energy levels are lowest when rivers meet the sea or lake and slow down, causing deposition. Deposition features such as deltas can form in these places. The deposition also occurs when the volume of water decreases. It happens at the end of a flood or during a time of drought. For example, the slower moving water inside a river’s bend will have less energy and therefore drop its load, helping to create a meander.
A major deposition feature of the river is the flood plain in its lower reaches. This is made up of deposited sand and silt, which is known as alluvium. Floodplains are therefore very fertile and have supported large agricultural communities since ancient times.
The long profile is a slice through the river from the source to the mouth, as seen in this diagram. Rivers are essentially trying to erode to their base level. That is usually sea level or sometimes a lake that the river may drain into. It is because, without the force of gravity, water will not flow. Near the river’s source, often high in the mountains, a river will cut vertically to form a steep v-shaped valley. In these mountainous environments, a river will often flow over a series of rapids and waterfalls. As rivers reach their mid-course, they continue to cut downwards but also cut laterally.
Finally, like a river near the sea, rivers flow over flat land. Most of the river’s energy is concentrated on cutting laterally and making the channel wider as it meanders its way towards the river’s mouth. The cross profile of rivers also changes.
Source of river water
There can be water in rivers even weeks after the last rains. So, where does that water come from? It turns out that, especially in winter, when less water evaporates through the plants. Some rainwater can infiltrate into the soil. But what happens when the water infiltrates downwards reaches the rocks that are under the topsoil? Well, It depends on what type of rock it finds.
Some rocks contain a lot of small holes, even though they are sometimes so small. These little holes are called pores, and rocks that have many pores are said to be porous. In general, when the rocks are porous, water can seep through the holes and continue its journey downward. But some rocks have very few pores. In this case, the rocks are said to be impermeable because water cannot pass through them. If a layer of impermeable rock blocks the downward passage of water, then water begins to collect in the porous rocks above.
This way, it can form large underground reservoirs of water known as groundwater aquifers. The uppermost level in which the pores contain water is called the water table. Rocks between the water table and the aquifer base can contain more than 20% of their volume of water. It means that more than 200 liters of water can be stored in each cubic meter of rock. A cubic meter is more or less the size of a fridge.
When it reaches an impermeable layer, the water can no longer continue to infiltrate downward. So it slowly moves, more or less following topography towards places where the water table is the lowest. In some places, the water table might come to the surface. In this case, the water can come out from underground and return to the surface. It is in this way that the rivers receive a lot of their water.
The level of the water table rises and falls with the rain and the seasons. But as long as the level does not drop lower than the river bed, water will seep up through the river bed. And the river can continue to flow even when it is not raining. But that’s not all. Many rivers start their course high in the mountains, where the peaks are covered in snow and ice.
In spring and summer, these rivers receive a lot of water, which forms when snow and ice melt. For this reason, in the Alps, there is often much more water in rivers in summer than in winter.
- Some of the water reaches the river as surface runoff when it rains.
- Some of the water comes from groundwater reservoirs.
- Finally, some of the water comes from the melting of snow and ice.
The Nile River is the longest river on earth. It measures around 4132 miles. It runs through or along the border of 10 other African countries. Over vast periods the primitive oceans formed. The water remained gas until the earth cooled below 212 degrees Fahrenheit. About 3.8 billion years ago, the water condensed into rain which filled the basins known as our oceans. We have five oceans that are all connected, namely the.
- Arctic ocean.
- Southern ocean.
- Indian ocean.
- Atlantic ocean.
- Pacific ocean.
Near the source, river channels are narrow and shallow due to the steepness of the terrain. Rivers in these environments contain many large boulders and rocks. With all the sediment on the riverbed, there is much turbulence and friction, slowing the water down. When the river has reached this lower course, the river flows fastest until it slows down on meeting the sea. Here the channels are deep and wide, and the banks are the smoothest.
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Hawkes, H.A., River zonation and classification. River ecology. Blackwell. pp. 312–374.
Cave, Cristi. “How a River Flows”. Stream Biology and Ecology.
Rosenberg, Matt, “Do All Rivers Flow South?”.