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- Importance of rivers
- Distribution of rivers in nature
- Drainage patterns
- Geometry of river systems
- Streamflow and sediment yield
- Rivers as agents of landscape evolution
- Alluvial fans
- The river system through time
World distribution of waterfalls
The distribution of waterfalls is not uniform, and large parts of the world are free of any notable occurrence. This is not surprising in view of the relatively large proportion of the Earth’s land area that consists of deserts and semiarid areas; these are understandably devoid of modern falls on climatic grounds. Ice-covered polar regions and relatively unbroken, low-lying plains and plateaus also are unfavourable sites of development.
Considered on a global basis, waterfalls tend to occur in three principal kinds of areas: (1) along the margins of high plateaus or the great fractures that dissect them; (2) along fall lines, which mark a zone between resistant crystalline rocks of continental interiors and weaker sedimentary formations of coastal regions; and (3) in high mountain areas, particularly those that were subjected to glaciation in the recent past.
Notable falls along high plateaus include the world’s highest, Angel Falls of the Churún River, Venezuela, with a drop of 979 metres and overall relief of more than 1,100 metres; Tugela Falls, issuing from the Great Escarpment, South Africa, which is 948 metres in height; Victoria Falls (108 metres) on the Zimbabwe-Zambia border; and Kalambo Falls (427 metres) on the Tanzania-Zambia border. The volume of flow at Victoria Falls is relatively large, approximately 1,080 cubic metres per second, but Guaíra Falls, a series of falls that until their submergence by the waters of Itaipú Dam in 1982 totaled 114 metres along the Paraná River, Brazil-Paraguay, had the largest known average discharge—13,300 cubic metres per second. During flood stages, however, even this figure is exceeded at some falls along the Orange River and elsewhere. Angel Falls, Iguaçu Falls (82 metres), in Brazil, and several others occur along the margins of high plateaus, east of the Andes, between Venezuela and Argentina.
Waterfalls that occur along fall lines are in some cases relatively indistinguishable from plateau examples—the Aughrabies Falls (146 metres), for instance, which occur where the Orange River leaves resistant crystalline rocks of the plateau in southern Africa. The typical fall-line example, however, occurs at the junction of the crystalline rocks of the Appalachian Mountains and the sedimentary coastal plain along the eastern United States. A number of major cities, including Philadelphia, Baltimore, and Washington, D.C., are a geographic consequence of the existence of falls along this line or zone because they present barriers to further inland navigation. In England there is an analogous example with respect to the line of towns including Cambridge that borders the Fens. The most spectacular fall-line waterfalls, however, include Churchill (formerly Grand) Falls, Labrador, Canada (75 metres); Jog Falls (Gersoppa Falls), Karnataka, India (253 metres); and Paulo Afonso Falls, Brazil (84 metres).
The last category, mountainous and formerly glaciated regions, include such well-known waterfalls as Yosemite Falls, California (739 metres), with a three-section drop; Yellowstone Falls, Wyoming (94 metres), with a two-section drop; Sutherland Falls, South Island, New Zealand (580 metres); and Krimmler Waterfall, Austria (380 metres). Other falls of considerable height or volume of flow occur elsewhere in mountainous and formerly glaciated regions—namely, in the Alps, the Sierra Nevada and northern Rocky Mountains of North America, and South Island, New Zealand. The ice-free parts of Iceland and the fjord (drowned-valley) region of Norway also should be cited. Both areas contain numerous falls by reason of suitable topography and climate. Australia also has a few falls, notably the Wollomombi, in the Great Dividing Range, New South Wales (482 metres).
Types of waterfalls
The several types of waterfalls that occur in nature may be classified according to a variety of schemes. One of the simplest of these is based on principal region of occurrence—high plateaus, fall lines, and formerly glaciated mountains, as discussed above. More meaningful, however, is an alternate, threefold classification system that places more emphasis on the specific ways in which geologic and physiographic conditions produce and affect waterfalls. Thus, falls can be categorized as: (1) those attributable to natural discordance of river profiles, whether caused by faulting (vertical movements of the Earth’s crust), glaciation, or other processes; (2) those attributable to differential erosion, which occurs whenever weak and resistant rocks are juxtaposed in some way; and (3) those attributable to constructional processes that create barriers and dams, over which water must fall. These three basic types will be discussed in turn.
Falls attributable to discordance of river profile
In one sense, all falls must be attributable to a discordance of river profile by their very definition. This category is here arbitrarily restricted, however, to exclude profile breaks that are caused by differential erosion and constructional processes. Remaining are waterfalls along fault scarps, uplifted plateaus and cliffs, glacial features of several kinds, karst topography—the caves and cave systems produced by solution of carbonate rocks—and falls that result from the issuance of springs from canyon walls high above valley floors.
The enormous rigid plates that make up the outer shell of the Earth continually move relative to one another, resulting in seafloor spreading, continental drift, and mountain building (see plate tectonics: Mountain building). These large-scale motions cause a buildup of strain within the rocks of the crust at some depth below the surface. Ultimately, the rocks must yield or shift in order to release this strain, and, when they suddenly do so, an earthquake results. Commonly, there will be some visible evidence of this sudden release at the Earth’s surface, perhaps manifested by the creation of a cliff or series of cliffs along a line or zone. The sloping surfaces that form the cliff fronts are called fault scarps. The vertical movements that produce fault scarps seldom amount to more than about three metres during an individual earthquake. Repeated faulting along the same line or zone, however, can produce scarps that are thousands of metres in height in relatively brief periods of geologic time. Waterfalls occur where the faults cross established drainage systems. The ultimate height of such falls depends not only on the total height of uplift but also on the rate of downcutting by the affected rivers. Rates of uplift tend to exceed rates of downcutting considerably in those parts of the world where uplift is ongoing today. Hence, it is normal for high waterfalls to exist due to uplift in many areas. In addition, some plateaus are produced by broader, regional uplifts that are relatively continuous and are not associated with earthquakes. The heights attained are nevertheless comparable after suitable time intervals. Major rift (fracture) systems of continental or subcontinental scale, some sea cliffs, and other features of this nature also are attributable to some form of faulting. All of them provide suitable sites for waterfall development.
The processes of glaciation have served this same end. Mountain ranges that formerly were glaciated contain falls at the outlets of cirques, bowl-shaped depressions in the headwaters of drainage areas that were formed by the accumulation of ice and its erosive action on the underlying bedrock. In addition, waterfalls are most common where hanging valleys occur. Such valleys generally form when glacier ice deeply erodes a main or trunk valley, leaving tributary valleys literally hanging far above the main valley floor. After the glaciers have melted and withdrawn, streams from such tributary valleys must fall in order to join the main valley drainage system below. Hanging valleys also can occur in response to faulting and in some other non-glacial situations: the chalk cliffs of England, for example, where small streams cannot cut downward with sufficient rapidity to keep pace with backwearing of the cliffs by marine erosion.
Other features that may result from glaciation include glacial potholes and glacial steps. The former are thought to originate principally as a result of the plastic flow of ice at the base of a glacier; this permits the gouging of semicylindrical holes in the bedrock beneath the path of flow. The holes or depressions are subsequently enlarged and deepened by meltwater runoff that is heavily laden with gravels, and they have become the sites of modern cascades in many instances.
The steps (or glacial stairway, as this feature is sometimes called) consist of treads and risers on a relatively giant scale that have been produced by the passage of ice over bedrock, particularly when alternating rock properties or joints offer differential resistance to the flow of ice. Again, the establishment of runoff after wastage of the ice has occurred will lead to a series of waterfalls or cascades at the site of each riser in the stairway.
Most spectacular among glacial features, however, are the overdeepened valleys along formerly glaciated coasts, as in Norway. These fjords are intimately associated with falls because the valley walls typically are both high and steep and because hanging valleys are ubiquitous.
Like the potholes mentioned above, the solution of limestones and other carbonate rocks leads to the formation of pits, sinks, caves, and interconnected systems of caverns, which together are termed karst topography. Terrain of this kind commonly contains water in many of the included passages in the form of standing pools, streams, and, where discontinuities of cavern levels occur, waterfalls. There are a few parts of the world where karst topography and its associated drainage are prominent features of the landscape, but, on the whole, falls attributable to cave-forming processes are not numerous. Springs that issue from canyon walls high above main valley floors are in the same category. Most of these artesian (free-flowing) systems result from the same type of solution phenomenon along joints and fractures that produce caves in carbonate rocks.