Waterfalls

Waterfalls are created by the free flow of a stream over a discontinuity in the streambed. The fall may be perpendicular and free from the rock surface, or it may run across the rock, frequently in a series of small falls, with the water essentially in free fall under the force of gravity. Waterfalls attract viewers for their scenic and aesthetic qualities. However, they also possess scientific value as an aid in the interpretation of Earth's history and economic importance as possible sites for the generation of hydroelectric power.

For many reasons, waterfalls remain vital to Earth and its residents in the twenty-first century. They are essential for maintaining healthy ecosystems with diverse plants and animals, and they continue to be a source of water that is easily purified for drinking. Integral to many economies, they promote tourism and are also an integral part of the culture of many groups of people and nations. Waterfalls reveal information about Earth’s past processes, which can inform scientists of its future. Finally, waterfalls promote plant growth and can mitigate some of the effects of anthropomorphic climate change.

Characteristics

Waterfalls occur at breaks in the relatively smooth profiles or streambed slopes of streams. The break in slope may be minor and result in only a riffle or slight roughening of the water surface. All streams possess riffles resulting from the hydraulic action that permits streams to carry sediment. Rapids result from a more profound interruption of the profile, often sufficient to block the passage of most boats. Rapids indicate that the streambed has encountered erosion-resistant bedrock, boulders, or other obstructions. Some purists maintain that true waterfalls exist only where water falls free of its streambed, plunging downward through the air. However, except for the largest and most spectacular falls, free fall is generally a function of the amount of discharge in the stream channel, varying with weather and climate, and the streambed's topography.

Most of the world’s streams have dry spells every year, during which a free-falling waterfall may become a cascade, a cataract, or even rapids. The Great Falls of the Potomac River dried up to very weak rapids during a late 1960s drought. However, it was still an obstruction to navigation and a potential site for waterpower development. It also possessed scenic appeal and presented evidence to geologists of the underlying rock structure and its place in deciphering Earth’s history. These are the primary reasons why people are interested in waterfalls, whether they are a thousand meters high or less than ten and whether they are free-falling or are better described as cascades, cataracts, or rapids.

Formation

On their route to the sea, lake, or some other termination, streams cut downward through loose sediments and bedrock by eroding and transporting some of this material as their sedimentary load. If the materials through which they are eroding are relatively soft or weak, the stream ultimately will achieve grade, denoted by a smooth, upwardly concave profile. Waterfalls indicate that something has prevented the stream from reaching an equilibrium between its discharge, or quantity of flow, and the sediment load it carries. The cause may lie in the structure of the underlying rocks, with particularly resistant layers encountered in the downward course of erosion. Frequently, the stream cannot achieve a smooth profile because of some event in geologic history, such as rapid tectonic uplift or glacial deepening of a major valley. Most surface relief is a consequence of tectonic activity in the past few million years, particularly the uplifting of terrain. This length of time is insufficient for streams to achieve even quasi-equilibrium or grade in the world's highlands, where complex geology often exposes rocks of varying resistance to weathering and stream erosion. Resistant rock layers may be sandstones, as in much of the Appalachian Mountains and Colorado Plateaus, or dolomite, such as the Niagara Escarpment, over which the Niagara River falls.

However, the effects of simple erosion are insufficient to describe waterfalls' formation and evolution. For example, the geologic history of the Niagara River and Niagara Falls is more complicated than merely encountering a resistant dolomite rock layer by the downward-cutting river. Continental glaciation of the eastern and central United States and Canada obliterated the pattern of preglacial stream drainage, and the roughly twelve thousand years that Niagara Falls has existed is insufficient time for that stream to achieve grade. Similarly, the head of navigation on the Mississippi River is at St. Anthony Falls in Minneapolis, Minnesota; upstream of the falls, the Mississippi flows on a new, postglacial course. The head of navigation of the Ohio River is the falls at Louisville, Kentucky, only a short distance upstream from the mouth of the Ohio, where it joins the Mississippi. This peculiar situation led geologists to search for the preglacial Ohio River, discovered buried by glacial sediments in northern Indiana and Ohio. Niagara Falls, St. Anthony Falls, and the falls of Ohio at Louisville may be considered large and dramatic knickpoints, or breaks in the smooth stream profile. Knickpoints migrate upstream as erosion proceeds toward grade or quasi-equilibrium, eventually becoming smaller and smaller until they are eliminated as significant breaks in the stream profile.

Waterfalls, therefore, represent unusual and temporary geologic circumstances and contribute to scientific knowledge in terms of explanations for each individual waterfall as well as a broader understanding of the geologic history of a region. Most waterfalls worldwide undoubtedly occur in younger, recently uplifted mountains subjected to alpine glaciation. Just as major rivers have greater eroding capability than their tributaries, larger valleys are occupied by larger glaciers capable of cutting deeper into the bedrock. The glaciers of tributary valleys are much smaller and have less potential for erosion than those in the main valleys. After the glaciers melt, the streams occupying the tributary valleys are left hanging, descending to the main stream as waterfalls. Hanging tributary valleys also occur in unglaciated uplands. Still, they are far more common in the spectacular scenery of alpine mountains, especially in the world’s major fjord regions: Alaska and British Columbia, Norway, Chile, and New Zealand’s South Island.

Scenic Appeal

The scenic appeal of waterfalls is why most people find them interesting. The sheer grandeur of the falling water inspires artists, photographers, and writers, whose products enhance the waterfalls’ fame and encourage multitudes to experience the view personally. The attraction of waterfalls is a significant factor in the decision to visit state and national parks and roadside waysides. Tourist counts are notoriously unreliable, but it is safe to say that millions have visited Niagara Falls, owing in part to its location near large population centers in the United States and Canada. By contrast, Victoria Falls on the Zambezi River in southern Africa, Angel Falls in Venezuela (the world’s highest, at 979 meters), the Iguaçu Falls on the border of Argentina and Brazil, and Guaíra Falls on the upper Parana River between Brazil and Paraguay all possess spectacular characteristics. Still, they are located too far from large centers of population to be visited easily.

There is also disagreement as to which waterfall has the greatest discharge. Unlike the height of a waterfall, which is static unless a catastrophic rockfall occurs, stream discharge varies throughout the year, from dry to wet years. Nearly all waterfalls occur in places with seasonal precipitation or in environments where the winter precipitation is in the form of snow, leading to spectacular flows during the snowmelt season but disappointing conditions when the stream is chiefly ice. Niagara retains an appeal when frozen, but the weather outside is generally uncomfortable at that time of year, and most tourists visit during the warm season. Thousands of people annually register disappointment at the appearance of Yosemite Falls and the other falls of Yosemite National Park during the long, dry summers. This is a characteristic of waterfalls in regions of alpine glaciation; they are frozen in winter and are most spectacular during the short snowmelt season of spring and early summer.

Tropical waterfalls, such as those of Africa, South America, and India, may have tremendous discharges during the rainy season but much lower flows during the dry season. One of Niagara’s great advantages as a tourist attraction is that it has huge natural reservoirs upstream in the form of the Great Lakes, which ensure an even discharge year after year, wet season and dry.

Rating Systems

Arguments concerning which waterfall among the giants is greatest frequently involve rating systems employing the height of the falls, its width, its discharge, and other quantifiable factors. Curiously, vista, or the opportunity to view the falls from a particular point, is often overlooked. Whereas Niagara Falls, for example, can be observed from several viewpoints, all of which overwhelm the observer with the view of a tremendous amount of water cascading over the edge of the escarpment into a broad open basin, the much wider Victoria Falls, in Africa, descends into a complicated narrow chasm or canyon, and little of the falls can actually be seen from one point on the surface. This is also true of Iguaçu Falls, which is up to several kilometers wide and consists of as many as twenty cataracts. Aircraft flights are the best way to observe these giants and are, in fact, the only way to experience more than a small fraction of those falls.

A hypothetical rating system may be required to incorporate many points of consideration, such as the amount of time a waterfall is in full flow each year and its availability to the public as an attraction. In addition, aesthetic and highly subjective considerations may come into play, such as which waterfalls are the most beautiful, inspiring, or dramatic. These and other considerations can be important if government agencies must make decisions regarding protecting a particular waterfall. Such scenarios confront many water projects, which must procure a favorable ratio of benefits to costs to survive. The allocation of limited financial resources increasingly involves such decisions.

Hydroelectric Power

The height of waterfalls creates another area of interest: the potential for hydroelectric power generation. “Head” is the term used to describe the difference in elevation between the water level at the top of a fall and that at the bottom. The higher the head, the greater the potential energy of the position, which can be converted into electrical energy as the water descends. It must be emphasized the waterfall itself is not harnessed; rather, the difference in water levels, or the head, is harnessed by diverting water from the upper level to flow downward through penstocks to the lower level. The potential energy of the water at the upper level is thus converted to kinetic energy, and the momentum of the moving water is used to drive hydroelectric generators.

Niagara Falls was the world’s largest single hydroelectric generating facility for years, and it is still among the largest hydroelectric projects. Most of the waterfalls of the tropics have a potential for power generation, but this has not yet been developed extensively. Europe, Japan, the United States, and Canada have the most fully developed hydroelectric generating capacities. Regions of alpine mountain glaciation have particularly high potential. The economic benefits of this aspect of waterfalls can be enormously significant locally and frequently nationwide. However, highly industrialized nations with significant numbers of automobiles demand far more energy than waterpower can generate. Most hydroelectric power generation today is not from natural waterfall sites but from what can be called "artificial waterfalls:" creating a head by constructing a high dam within a narrow valley or gorge. In the mid-2020s, China, Brazil, and Canada were the leading countries in the use of hydroelectric power.

At the beginning of the Industrial Revolution, before the development of electricity and its distribution through transmission lines, water-power sites provided energy for countless small factories and mills. Many of these were the sites of small waterfalls and rapids, such as along the fall line of the Piedmont in the southeastern United States. The Great Falls of the Potomac are an example of such a site. Developing electrical transmission lines freed industry from these locations near falls and rapids and allowed expansion throughout the countryside. The smaller waterpower sites fell into disuse, and only a few have been preserved for historical reasons. The larger sites, such as Niagara Falls, remain essential to the economy.

Principal Terms

cascade: a small waterfall or series of small falls

cataract: most frequently, an overwhelming flood or a great volume of flow over a cliff; sometimes used interchangeably with “cascade”

grade: a hypothetical uniform profile of a stream seeking to achieve quasi-equilibrium; the slope of the streambed between its highest and lowest points

knickpoint (or nickpoint): an abrupt change in the stream profile, generally caused by a resistant rock layer that retards the rate of erosion

rapids: a turbulent flow in a stream caused by obstructions and constrictions of the channel or by resistant rock layers

riffles: a smaller version of rapids, a turbulent flow between calm pools, found in nearly all streams for hydraulic reasons

tectonics (tectonic activity): vertical or horizontal movements in Earth’s crust, displacing rocks, landforms, and stream gradients

Bibliography

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