A waterfall can be defined as an area where a flowing river or water body abruptly and nearly vertically drops. Waterfalls can also be said to represent major interruptions in the flow of any river.
In most cases, we know that rivers tend to smooth out irregularities in the land by a complex process of erosion and deposition. With time, the long profile of a river, which is also known as the graph of its gradient, develops into the shape of a smooth curve. This curve is steepest towards the source of the river and is the gentlest towards the mouth. It is the characteristic of waterfall geology to interrupt this particular curve.
The presence of waterfalls is also a measure of the progress of erosion. There are some experts who refer to waterfalls as simple falls or cataracts. It should be noted that waterfalls are usually called cataracts when large volumes of water are present.
Students might also like to know that waterfalls that have a small height and lesser steepness are known as cascades. The term ‘cascades’ is also applied to a series of small falls that run along a river. Further, rapids are known as the gentler reaches of rivers that often show turbulent flow and white water. This turbulent flow and white water are in response to some local increase in channel gradient.
Also, you might find it fun to learn that the highest waterfall in the world is known as Angel Falls. It is located in Venezuela and measures up to 807 meters or 2,650 feet in height. The largest waterfall, on the other hand, is known as the Chutes de Khone or the Khone Falls. The Khone Falls is located on the Mekong River in Laos. The total volume of water passing over that waterfall has been estimated at 410,000 cubic feet or 11,600 cubic meters per second. This is shocking especially considering the fact that the height of the Khone Falls is only 70 meters or 230 feet!
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Conditions that Give Rise to a Waterfall
There are various conditions that could result in the rise of a waterfall. It is important for students to learn what those conditions are. And lucky for you, we have prepared a list of those conditions and that list is mentioned below.
1. The Difference in Rock Types
This is one of the most common conditions that give rise to a waterfall. We know that rivers cross several lithological boundaries. And if a river passes from a resistant rock bed to a softer rock bed, then that would result in erosion of the soft rock with quicker speed. This also leads to a steepening of the gradient at the junction between the types of rock.
This condition also occurs when a river cuts and exhumes a junction between different types of rock beds. One example of this is the riverbed of Niagara Falls. Niagara Falls forms a part of the boundary between Canada and the United States of America. It has a blocky dolomite cap that is overlying a series of several weaker sandstones and shales.
2. Presence of Bars of Hard Rock
Another common cause of waterfalls is the presence of bars of hard rock in the riverbed. An example of this is the series of cataracts that have been created on the Nile. This has happened in the place where the river has worn its bed sufficiently to uncover the underlying hard and crystalline basement rock.
3. Structure or Shape of Land
There are also waterfalls that are not caused due to the character of rock formation. Instead, those waterfalls occur because of the structure or shape of the land. For example, uplifted plateau basalts can provide a resistant platform at the edge from which the rivers are producing waterfalls. One can actually see this occur on the Antrim basalts in Northern Ireland.
A large-scale example of this is the morphology of the southern half of Africa. There is a high plateau that is surrounded by a steep scarp slope. It creates waterfalls and rapids on most of the major rivers of that area.
One might find it interesting to note that this includes the Augrabies Falls on the Orange River and the Livingstone Falls on the Congo River. It should also be noted that usually the occurrence of waterfalls increases in mountainous terrains as the slopes get steeper.
4. Tectonic Movement
It should be noted that geology and erosion are not the only factors that result in the creation of waterfalls. Instead, tectonic movements also help bring hard and soft rocks together. This occurs when the tectonic movement is along a fault. It further encourages the establishment of a waterfall.
5. Drop-in Sea Levels
A drop in sea levels also promoted increased downcutting and the retreat upstream of a knickpoint. This means the sharp change of gradient that would indicate the change of a base-level of a river. Also, depending on the change of river flow, sea level, geology, and several other factors, it is possible for rapids or falls to develop at the knickpoint.
There are several waterfalls that have been created through glaciation too. This occurs in places where the valleys have become over-deepened by ice and the tributary valleys have been left high up on the sides of the steep valley. For example, in the glacially gouged Yosemite Valley in California, the Yosemite Upper Falls take a tumble of 436 meters or 1,430 feet from a high-hanging valley.
It is quite interesting to note that if one looks at the time scale of a river, then a waterfall is just a temporary feature. This means that waterfalls are eventually worn away. The rapidity of that erosion depends on several factors, including the height of a waterfall, the volume of the flow, the type of rocks, the structure of the rocks, and many other factors.
It is also possible in some cases that the site of the waterfall might migrate upstream. This can happen through headward erosion of a scarp of a cliff. In some other cases, erosion might also act downward. This would bevel the entire reach of the river that contains the waterfall.
As more time passes, by either or both of the above-stated means, the rivers might act on their inescapable tendency to eliminate any waterfalls that were formed. The energy of rivers is also directed towards the achievement of a comparatively smooth, upward concave, and longitudinal profile.
The level of energy available for erosion at the base of a waterfall is very large even in the absence of entrained rock debris. The entrained rock debris serves as a tool of erosion for the river.
Another defining feature that is associated with waterfalls of a larger size is the presence of a plunge pool. It is a basin that is scoured out of the river channel that is present beneath the falling water and it is present with respect to the volume of the water flow and the height of the waterfall.
In some cases, the depth of the plunge pool can also roughly equal the height of the cliff. This results in the formation of the waterfall. Students should remember that plunge pools can also cause the collapse of the cliff face. This results in the retreat of the waterfall.
The retreat of waterfalls is a very significant feature that one can observe in some places. For example, at Niagara, the falls have retreated a total of 7 miles or 11 km from the face of the escarpment where they began.
Currently, a large portion of the water of Niagara is diverted for hydroelectric power generation. However, it is estimated that if this was not the case, then the rate of retreat with the normal flow would be around 1 meter or 3 feet per year!
Fun Facts about the Waterfalls
Did you know that there are different types of waterfalls? If you want to learn about those types of waterfalls, then go through the list that is mentioned below.
1. Ledge Waterfall
A ledge waterfall is also known as the classic waterfall. This type of waterfall has a curtain of water that one usually pictures in his or her mind while thinking of a waterfall. This curtain of water partially touches the bedrock.
2. Fan Waterfall
This type of waterfall created the shape of a fan. It happens because the water is spread out horizontally as it descends.
A cascade is a type of waterfall that descends over the rocks that are similar to the shape of steps. These types of waterfalls are not as dangerous or steep as the other waterfalls. One can usually relax and play in these types of waterfalls.
A cataract is a strong, powerful, huge, and dangerous waterfall.
5. Punchbowl Waterfall
As the name indicates, a punchbowl waterfall has a very wide plunge pool at the bottom.
A chute is a type of very narrow waterfall. This type of waterfall usually forces water through it at very high pressure.
7. Plunge Waterfalls
As you might have guessed, plunge waterfalls quickly plunge over the ledge. These waterfalls don’t touch the hard rock surface.