Waterfalls: Forms, Distribution, Processes and Rates of Recession

Waterfalls occur on a huge diversity of rock types, although in general, they do not form persistent or large falls on soft or unconsolidated rocks. Table 2 suggests that they may be especially common on bedded sandstones and on basaltic lavas. However, they also occur inter alia on granites (Chisholm 1885), gneiss, schist, limestones, dolomites, quartzites and conglomerates.

The most venerable model for waterfall development with respect to rock type is the so-called caprock model that was described by Lyell (1845) in the context of the Niagara Falls. Lyell (1875: 355, 356) remarked that the uppermost rocks of the Falls consist of hard Silurian limestone around 30 m thick, beneath which lie soft shales of equal thickness, continually undermined by the action of the spray, which rises from the pool into which so large a body of water is projected and is driven violently by gusts of wind against the base of the precipice. In consequence of this action, and that of frost, the shale disintegrates and crumbles away, and portions of the incumbent rock overhang forty feet, and often when unsupported tumble down, so that the Falls do not remain absolutely stationary at the same spot, even for half a century. This model was accepted by Geikie (1893) and Gilbert (1895). Likewise, Cotton (1941: 32) reported that Falls, as distinguished from rapids, are developed by vertical corrosion where rivers cross the edges of outcropping strata of resistant rock, and especially where these are horizontal or only gently inclined, and where they overlie weak materials. The resistant beds, or fall markers, may be lava flows or indurated sedimentary strata. He gave the example of the Wairua Fall of the North Island of New Zealand, formed where the river has cut down through lava that had invaded the floor of its former valley. The lava rock itself, though free from joint cracks and resistant to erosion in its surface layer, is weakened below by the presence of tension joints due to shrinkage during cooling, the result being effective undermining of the edge of the superficial strong layer by plunge-pool erosion (Cotton 1941: 32, 33). The model has also been applied to High Force in northern England, where the River Tees falls over an outcrop of the dolerite Whin Sill, which lies above the softer Carboniferous limestone (Goudie and Gardner 1985: 18).

The caprock model is not of universal applicability as pointed out by Young (1985). Young et al. (2009: 202–204) wrote: The widespread occurrence of major waterfalls on sandstones is normally attributed to the undercutting of a caprock by the disintegration of a softer rock beneath it. However, instead of being undercut, many waterfalls in sandstones are buttressed at the base, and some even lack plunge pools. They believed that the basic requirement for the development of waterfalls appears to be the outcrop of rock that, because of its lithological and structural properties, will stand in a steep face. Basal undercutting may certainly promote the development of a steep face, but it is not an essential requirement. Moreover, as Von Engeln (1942) and others have pointed out, horizontal strata are not a sine qua non, for vertical barrier falls result from the presence of a barrier (e.g. of lava). Some of the greatest waterfalls, including the Victoria Falls and the Iguazu Falls, occur in tick basalt sequences, and the caprock model does not apply to them.

In this section, the links between rock type and water fall development are explored in the context of specific regional sites.

USA.Worcester (1939) stressed the importance of lavas as on the Shoshone, Twin and American Falls on the Snake River in Idaho, USA. Ray and Rahn (1997) looked at the waterfalls of Black Hills, Dakota. These are developed on Precambrian metamorphic and igneous rocks. Tarr (1905) described many waterfalls in central New York State associated with Devonian sandstones overlying shales.

South America. In South America, the great Iguazu Falls (Fig. 2) and others in the Parana basin (Lima and Binda 2013) have developed over Cretaceous basalts laid down across sandstones, while in Venezuela, some of the tallest falls in the world have developed over sandstones. In Amazonas (Brazil), falls have developed in quartz arenites (Nogueira et al. 2001), while in the Pernambuco area of northeast Brazil, falls have developed in Neoproterozoic granites, granodiorites and orthogneisses.

Fig. 2

Africa. In South Africa, Norman and Whitfield (2006) noted that in the east of the country, the Mgeni River plunges over Howick Falls, a precipitous 95 m drop over a resistant dolerite sill that intruded Ecca shales. Waterfalls are associated with the Great Escarpment as near Sabie – Horseshoe Falls, Lone Creek Falls (68 m) and Bridal Veil Falls (70 m). All owe their existence to rapidly eroding streams in the Sabie valley meeting with hard quartzites at the upper contact of the Malmani dolomite and the overlying Pretoria Group. Also, close to Sabie are the MacMac Falls (56 m high), the Lisbon Falls (92 m) and the Berlin Falls (150 m), which result from streams falling off platform-like Black Reef quartzites. The 145-m tall Augrabies Falls on the Orange River have formed in granite-gneiss (Tooth 2015) (Fig. 3). Further north, the Victoria Falls on the Zambezi are formed in Jurassic (Karoo-age) basalts (Clark 1952, Moore and Cotterill 2010) (Fig. 4). In Guinea, dolerite and gabbro sills are the cause of several waterfalls, where they form hard rock outcrops along river beds in the Fouta Djallon (e.g. the Kinkon Falls) (Buckle 1978: 54).

Fig. 3

Fig. 4

Australasia. In Australia, many of the waterfalls have developed in sandstones, as with the waterfalls of the Kimberley, which fall over Proterozoic rocks (Fig. 5). Seidl et al. (1996) noted that in southeastern Australia, on the New England Tableland, many waterfalls had formed in Paleozoic fine-grained meta-sedimentary rocks, as at Wollomombi, though some occurred in Late Carboniferous granites. Other falls in New South Wales have developed in rhyolites (Bishop and Goldrick 1992). In New Zealand, falls on the Waipaoa River in the North Island are formed on Cretaceous to Pliocene shelf and slope sediments (siltstones, mudstones and sandstones) (Crosby and Whipple 2006), while the Taranaki Falls are at the edge of an old lava flow. The Waipunga Falls, further east, flow over ignimbrite rock deposited in the 200 AD Taupō eruption. Both the Wairua Falls in Northland and the Bridal Veil Falls near Raglan drop over old basalt lava flows (Manatū Taonga MCH 2018).

Fig. 5

Asia and Pacific Basin. In India (Kale 2014), the Nohkalikai Falls on the Pyjngithuli River over the Meghalaya Plateau have a sheer drop of c. 198 m and have formed on the Eocene sandstones of the Cherra Formation. The Gersoppa or Jog Falls (c. 253 m high) occur on the Sharavathi River in Karnataka, where it leaps over the Western Ghats. It is underlain by banded gneisses. In Korea, Migoń et al. (2018) describe waterfalls developed in granites, while in northeast China, waterfalls have developed on volcanic rocks (Zhang et al. 2011) and on the limestones of the Yunnan–Guizhou Plateau (Hankui et al. 1984). The waterfalls of Central Vietnam are developed in basalt (Phuong et al. 2017). In the Pacific Basin, waterfalls are present on young volcanic rocks as in Japan (Hayakawa et al. 2008, Yoshida et al. 2017), Hawaii (Mackey et al. 2014) and French Polynesia (Ye et al. 2013).

Europe. In Poland, Alexandrowicz (1994) found that in the case of the Polish Outer Carpathian Mountains, small waterfalls developed on Flysch sandstones, while in the Czech Republic, many waterfalls in the Jizerské Hory region are formed in granites (Migoń 2016). In Iceland, waterfalls are formed in columnar basalts of relatively recent age, associated with mid-Ocean sea floor spreading (Schwarzbach 1967, Baynes et al. 2015) In Scotland, the falls in the Corrieshalloch Gorge are formed in Moine schists, the Falls of Clyde and the Grey Mare’s Tail in greywacke sandstones, and the Falls of Dochart in schist. In Wales, the Afon Rhaeadr Falls are developed in Ordovician slates, the Trymyn Falls in Silurian sandstones and the Hepste Falls and Mellte Falls also in sandstones. In England, the Lydford Gorge waterfall is formed in slates, High Force is on a dolerite sill overlying softer Carboniferous limestone and the Aysgarth falls on limestone interspersed with shale beds (Gregory 1997).

Many waterfalls occur in areas with lithological contrasts. Bloom (1998: 258) referred to this in the context of the Fall Line in the eastern USA: Such an erodibility contrast is to be seen in the eastern United States along the Fall Zone – a seaward dipping exhumed erosion surface between crystalline rocks of the Piedmont and New England Provinces and the overlying coastal plain sediments of Cretaceous and younger ages. Post-orogenic erosion created this surface of low relief before it was buried by onlapping marine sediments. Subsequent gentle epeirogenic warping has uplifted the landward region and depressed the seaward part so that a narrow bevel or facet of older metamorphic rocks has been exposed on the inland edge of the coastal plain by erosion of the former cover strata. The steepened gradients and many waterfalls on rivers where they cross the Fall Zone gave the region its name.

In general terms, rock joint and fracture characteristics are a significant control of fall morphology (Scott and Wohl 2019), and resistance and bed disposition affect the nature and location of step-pool sequences (Wohl 2000). Ortega et al. (2013) studied waterfalls in the Rocky Mountain National Park in Colorado, USA. They found that the shape of individual waterfalls and their height of drop correlated well with bedrock properties. Waterfalls in bedrock lacking vertical joints perpendicular to flow are more likely to have a single drop rather than multiple drops, and taller waterfalls correlate with more widely spaced horizontal joints or bedding planes. Likewise, Lima and Flores (2017) found that significant differences in the vesicularity and jointing of basaltic flows influenced the form of knickpoints in Parana basalts in Brazil. The role of stress relief and the associated development of vertical jointing also need to be considered (Lee 1978).

In the era of cyclic and evolutionary geomorphology, Davis (1884) doubted that falls would develop in catchments that had reached a state of maturity and averred that waterfalls are seldom found in old countries of flat rocks and moderate elevation. As Lobeck (1939: 197) recognised: Waterfalls and rapids are criteria of youth. There are two kinds: first, those which develop in the normal life history of a river and indicate that the stream has not yet acquired a graded slope; and, second, those which result from some disturbance, accident, or interruption in the life of the stream, imposed upon it by some outside force.

Major changes in base level with rift valley development provide ideal conditions for waterfall development. The classic example of this is provided by the Kalambo Falls at the border of Zambia and Tanzania (Buckle 1978). The Murchison Falls on the Nile occur where the river plunges over the pelitic schists of the Western Rift (Wolman and Giegengack 2008). Vertical waterfalls also occur along the Dead Sea Rift in Israel in a dolomite caprock with underlying, marly-limestone footrock (Enzel et al. 2005, Haviv et al. 2010). Malatesta and Lamb (2017) noted than in California, waterfalls commonly occur near the bounding faults of mountain ranges. Working in Death Valley, they found that incision of alluvial fans, resulting from climatic and tectonic forces, can expose waterfalls. Surface ruptures along the Chelungpu thrust fault in west-central Taiwan caused formation of knickpoints and small waterfalls according with bedrock exposure in riverbeds when the 921 Chi-Chi Earthquake occurred on September 21, 1999 (Hayakawa et al. 2009, 2010). Waterfalls also occur across the Main Boundary Thrust zone of the sub-Himalayas in northern India (Kothyari et al. 2010) and the Trans Himadri Fault of the Kumaun Tethys Himalaya (Kotlia, Joshi 2013).

Rivers in areas of tectonic uplift may not be able to cut down sufficiently quickly to have smoothly graded courses, and so may have knickpoints (Jansen et al. 2011) or may cascade over cliffs producing waterfalls. An example of this is provided by the coastline of California (Limber, Barnard 2018). Here, there is an active margin shoreline characterised by uplift, cliff retreat and river incision, with consequent formation of waterfalls. In Hawaii, the Ka’ula’ula waterfall has migrated backwards over the past 120 ka. It is a knickpoint that was initiated by sea-cliff erosion at a time of high sea level during the last interglacial (Mackey et al. 2014). In Tahiti, Ye et al. (2013) suggested that a sudden drop of sea level followed by cliffing created knickpoint conditions that led to waterfall formation. Waterfalls caused by cliff retreat and the creation of hanging valleys are also a feature of the north coasts of Devon and Cornwall in southwest England (Arber 1911).

Waterfalls are widespread in areas that were glaciated in the Pleistocene. Notable here are the waterfalls that cascade down the sides of fjords in Norway and New Zealand (Fig. 6) and of glacial troughs in the European Alps (Hayakawa 2011), the Pyrenees (Ortega-Becerril et al. 2017) or the flanks of Yosemite in the USA (Waltham 2012). These result from the formation of hanging valleys and overdeepened trunk valleys. However, in addition, as Russell (1898: 61) pointed out, many of the falls in the drift-covered region of North America are due to the turning of streams from pre-glacial valleys in such a way as to cause them to flow over what were formerly divides or rocky spurs between adjacent streams and plunge into valleys. Great Falls, Connecticut, is an example of a feature created by such glacial activity. Waterfalls can occur on cirque headwalls, where these undercut pre-glacial plateaus – like these in the Karkonosze Mountains of Poland.

Fig. 6

Large rockfalls and landslides can dam stream channels, leading to the development of lakes from which outflow may occur in the form of a waterfall over the downstream face of the deposit. This was recognised as a waterfall type by Lobeck (1939), as shown in Fig. 1, but relatively little work has been conducted on them. However, examples are known from steep mountain ranges, particularly in areas with seismic activity, as with the Karakoram Mountains. Wang et al. (2014) discuss this in the context of the Wenchuan earthquake in China in 2008.

Lava dams can block rivers, and thereby create lakes and waterfalls. Examples of this are known, for example, from the San Francisco Volcanic Field in Arizona (Plescia 2008), the North Island of New Zealand (Cook et al. 2018) and the Kegon Falls of Japan (Hayakawa 2013).

Some continental margins are lined by great escarpments that have developed on passive plate margins and which may have become elevated, at least in part, by faulting, thereby promoting river incision and gorge development. This is the case with eastern Australia (Seidl et al. 1996, Weissel and Seidl 1997), the Western Ghats of India (Kale 2010), the Serra do Mar in Brazil (Stevaux, Latrubesse 2010) and the western and eastern margins of southern Africa. In the last case, falls are notable on the Kunene River (at Ruacana), on the Orange (at Augrabies) and on the Tugela.

As escarpments retreat, streams on the plateau top may have their catchment areas and flows reduced, while streams eating backwards have their drainage areas and flows increased. This means that the former become less powerful, while the latter become more powerful. This can lead to an acceleration of gorge incision (Berlin, Anderson 2007). The smaller drainage catchments become unable to keep up with the incision of the main stream, and so steep knickpoints and hanging valleys develop (Crosbie, Whipple 2006, Wobus et al. 2006).

Any change in relative base level, which might not involve river capture, sea-level change or glaciation, has the potential to result in headward migration of knickpoints. This was demonstrated in the context of the Colorado Front Range by Anderson et al. (2006). However, river capture can lead to base level changes that are conducive to stream incision and waterfall development. For instance, a possible explanation for the development of the Victoria Falls is that the Upper Zambezi was captured by a headwater tributary of the middle Zambezi relatively recently in the late Cenozoic. The rapid headward erosion of the Batoka and higher gorges towards the falls would have been initiated by the marked lowering of base level following the capture (Moore, Cotterill 2010). River capture has also been implicated in the development of falls across the Kunene River at Epupa and Ruacana in Namibia/Angola (Kanthack 1921, Wellington 1955: 65). In Britain, the Lydford Gorge Falls are a classic example of the role of river capture (Gregory 1997).

It is possible that some gorges and amphitheatrical valley heads, which become the sites of waterfalls, were initiated by past megafloods. Lamb and Dietrich (2009) postulated that this was the case in the volcanic terrains of NW USA, where catastrophic floods (e.g. the Bonneville Flood) have carved steep, blunt-headed canyons in columnar basalt. Likewise, Lamb et al. (2014) argued that the Malad Gorge in Idaho, which has been cut into columnar basalt, was not caused by normal fluvial erosion or by groundwater seepage. Rather, it was due to a megaflood at 46 ka when lava flows dammed the Wood River, resulting in outburst flooding. The glacial Lake Missoula floods generated huge waterfalls, such as the Palouse Falls in Washington State (Waltham 2010), and breaching of a chalk barrier in what is now the English channel by overflow from a proglacial lake created enormous waterfalls (Gupta et al. 2017). Waterfalls associated with glacial megafloods are also known from NW Germany (Meinsen et al. 2011) and the Altai Mountains of Siberia (Rudoy 2002). Large floods in the Holocene have greatly influenced canyon evolution in Iceland (Baynes et al. 2015), and massive glacial floods may have contributed to the formation of large waterfalls in the Tsangpo Gorge in Tibet (Montgomery et al. 2004). Floods derived from caldera breaching have caused waterfalls in northern Japan (Kataoka 2011). Some falls that are currently largely dry may have been subjected to much larger flows in the Pleistocene, as was the case with the so-called Dry Falls in Washington State and with Malham Cove in Yorkshire, England, though it turned into a waterfall for the first time in living memory during the exceptionally wet winter of 2015/2016 (McCarthy et al. 2016).