Waterfalls are widespread fluvial landforms that have been described from many parts of the world and have attracted the attention of great geomorphologists such as Charles Lyell, Grove K. Gilbert and William M. Davis. They are currently of great interest because of their scenic and geodiversity value. Waterfalls are also important geomorphologically because they are an extreme manifestation of a knickpoint/channel gradient steepening. That said, as Young (1985) pointed out, they have been the subject of surprisingly limited research. The purpose of this paper is to review such research that has been undertaken, to classify waterfalls, and to try and establish the major factors that have controlled their form, distribution and rates of retreat. Such factors include climatic conditions, rock types, the history of glaciation and of climatic change, and tectonic situation.
These very beautiful features have been described by many travellers and explorers (e.g. Curzon 1923, Rashleigh 1935) and have drawn the attention of poets and artists (Hudson 2012, Hayman 2014, Cole 2015). They have been listed in various popular series (e.g. Reader’s Digest 1980, 1993) and on some comprehensive websites (e.g. World Waterfall Database 2018, European waterfalls 2018). Thirty-eight World Heritage Properties also include waterfalls in their designation (WHC 2018) (Table 1). There are also many waterfalls listed in the World Heritage Tentative Lists, including, for example, in the Aberdare Mountains in Kenya and Vatnajökull in Iceland. In addition, some waterfalls are actual or potential geomorphosites, as in India (Kale 2014), Brazil (Santos et al. 2015), Malaysia (Tongkul 2016) and Spain (Ortega-Becerril et al. 2017). In Britain, many waterfalls have been designated as Geological Conservation Review sites and include the following: Alport Valley, Aysgarth, Corrieshalloch Gorge, Falls of Clyde, Falls of Dochart, Grey Mare’s Tail, Hepste, Llugwy, Lydford Gorge, Mellte, Rhaeadr and Twymyn (Gregory 1997).
World Heritage Sites with waterfalls included in their designation.
World Heritage Site | Country |
---|---|
Wet tropics of Queensland | Australia |
Greater Blue Mountains Area | Australia |
Purnulu National Park | Australia |
Iguacu National Park | Brazil |
Atlantic Forest South East Reserves | Brazil |
Central Amazon Conservation Complex | Brazil |
Pirin National Park | Bulgaria |
Nahanni National Park | Canada |
Canadian Rocky Mountain Parks | Canada |
Gros Morne National Park | Canada |
Mount Huangshan | China |
Huanglong Scenic and Historic Interest Area | China |
Jiuzhaigou Scenic and Historic Interest Area | China |
Wulinghuan Scenic and Historic Interest Area | China |
Lushan National Park | China |
Mount Sanqingshan National Park | China |
China Danxia | China |
Cocos Island National Park | Costa Rica |
Talamanca Range-La Amistad Reserves / La Amistad National Park | Costa Rica / Panama |
Plitvice Lakes National Park | Croatia |
Morne Trois Pitons National Park | Dominica |
Sangay National Park | Ecuador |
Rio Platano Nature Reserve | Honduras |
Tropical Rainforest Heritage of Sumatra | Indonesia |
Gunung Mulu National Park | Malaysia |
Te Wahipounamu – South West New Zealand | New Zealand |
West Norwegian Fjords | Norway |
Laurisilva of Madeira | Portugal |
Putorana Plateau | Russian Federation |
Škocjan Caves | Slovenia |
Jeju Volcanic Island and Lava Tubes | South Korea |
Dong Phayayen-Khao Yai Forest Complex | Thailand |
Thungyai-Huai Kha Khaeng Wildlife Sanctuaries | Thailand |
Hierapolis-Pamukkale | Turkey |
Ruwenzori Mountains National Park | Uganda |
Yosemite National Park | USA |
Canaima National Park | Venezuela |
Mosi-oa-Tunya/Victoria Falls | Zambia and Zimbabwe |
Concerns have been expressed that many impressive waterfalls are being obliterated or diminished by the damming or diversion of rivers, as with the Guairá Falls, located on the Paraná River at the border of Brazil and Paraguay. They disappeared when submerged by dam construction in 1982, after having been one of the most powerful falls in the world (Niland 2017). Likewise, the Ripon Falls at the exit of the Nile from Lake Victoria were effectively eliminated in 1954 by the construction of the Owen Falls dam, while in Norway the flow over the Tyssestrengene Falls, following their use by the Norwegian Hydroelectric Power Authority, has diminished to such a point that only after heavy snow melts is there any flow of substance. Most of the year, there is no water flowing. Likewise, hydropower development threatens Estonia’s major waterfall (Ehrlich, Reimann 2010) and has destroyed various waterfalls in Jamaica (Hudson 1999).
Waterfalls have considerable ecological importance, in that they not only act as barriers to the movement of organisms, but also provide specific habitats of conservation value (e.g. Hora 1932, Clayton et al. 2016). Deposits associated with their plunge pools can be used to establish past precipitation events and trends (Nott, Price 1994, Nott et al. 1996).
Ford (1968: 1219) provided a definition of waterfalls and related phenomena: A A A A
There has also been considerable debate as to which waterfalls are the largest, and whether this should be based on the height of the largest vertical fall, the combined height of all falls at a site, the width of the site or the discharge of the flow over the fall (Matthes 1922, Plumb 1993, Mabin 2000). Table 2 lists the world waterfalls in terms of their heights. The world waterfall database documents 949 waterfalls between c. 100 and 1000 m high and ranging in estimated discharge from c. 150 to 42,500 m3 s−1.
Heights of world waterfalls with fall of more than 165 m.
Fall | River | Country | Height of fall (m) | Setting | Geology |
---|---|---|---|---|---|
Kerepakupi (Salto Angel) | Rio Gauja | Venezuela | 807 | Plateau edge | Sandstone |
Kukenaam, salto | Rio Kukenaam | Venezuela | 674 | Plateau edge | Sandstone |
Ventisquero Colgante | – | Chile | 549 | Glacial | – |
Gocta | Cocahuyaco | Peru | 540 | Amazonia | – |
Ribbon | Ribbon Creek | California, USA | 491 | Glacial | Granite |
Mtarazi | Mtarazi | Zimbabwe | 479 | Plateau edge | Granite/dolerite |
Cerberus | Ice Fall Brook | Canada BC | 475 | Glacial | Volcanics |
Piedra Bolada | Piedra Bolada | Mexico | 453 | Rift margin | Granite |
Yosemite Falls | Yosemite Creek | California, USA | 436 | Glacial | Granite |
Tugela | Tugela | South Africa | 411 | Plateau edge | Sandstone |
Sju Søstre | Knivsflåelvane | Norway | 410 | Glacial | Misc. crystalline |
Blanche | – | Réunion | 400 | Volcanic | Lava |
Churún Vena | – | Venezuela | 400 | Plateau edge | Sandstone |
Castaño Overo | Castaño Overo | Argentina | 366 | Glacial | Volcanic |
El Dorado | Rio Aracá | Brazil | 353 | Plateau edge | Sandstone |
Nohkalikai | Pyjngithuli River | India | 340 | Plateau edge (horst) | Eocene sandstones |
Mardalsfossen | Inste Mardøla | Norway | 320 | Glacial | – |
Skytjefossen | Skytjedalen | Norway | 315 | Glacial | – |
Turner | Cleft Creek | New Zealand | 314 | Glacial | – |
Tyssestrengene | Tysso | Norway | 312 | Glacial | – |
Basaseachic | Basaseachic | Mexico | 312 | Rift margin | – |
Roraima, Salto | – | Venezuela | 305 | Plateau edge | – |
Trou de Fer | Bras de Caverne | Réunion | 305 | Volcano | Volcanics |
Staubbachfall | Staubbach | Switzerland | 297 | Glacial | Sandstone |
Gavarnie | Gave de Pau | France | 281 | Glacial | Lava |
Vetisfossen | Morka-Koldedøla | Norway | 275 | Glacial | – |
Volefossen | – | Norway | 274 | Glacial | – |
Arpenaz | La Laiteuse | France | 270 | Glacial | – |
Sutherland | Arthur | New Zealand | 270 | Glacial | – |
Tueeulala | Falls Creek | USA | 268 | Glacial | – |
Wallaman | Stoney Creek | Australia | 268 | Plateau edge | – |
Parijaro | Rio Alto Cutiverini | Peru | 267 | Glacial | – |
Kyrfoss | – | Norway | 265 | Glacial | – |
Hunlen | Hunlen Creek | Canada BC | 260 | Glacial | Limestone |
Takkakaw | – | Canada BC | 260 | Plateau edge | Quartzite and conglomerate |
King Edward VIII | Semang | Guyana | 256 | Plateau edge | – |
Jog | Saravati | India | 253 | Plateau edge | Banded gneiss |
Skrikjo | Skrikjo | Norway | 250 | Glacial | – |
Gjerdefossen | Ktituervla | Norway | 245 | Glacial | Sandstone and conglomerate |
Kaieteur | Potaro | Guyana | 226 | Plateau edge | Metasediments |
Wollomombi | Wollomombi | Australia NSW | 224 | Plateau edge | Sandstone, quartzite and shales |
Kalambo | Kalambo | Zambia and Tanzania | 221 | Rift edge | Sandstone, quartzite and shales |
Tad Fane | – | Laos | 213 | Plateau edge | – |
Kjerrskredfossen | Kjelfossgrovi | Norway | 206 | Glacial | |
Kjelfossen | – | Norway | 198 | Glacial | |
Bridalveil | Bridalveil Creek | California, USA | 189 | Glacial | |
Drury Falls | Fall Creek | USA | 183 | Glacial | |
Svøufallet | Grødola | Norway | 167 | Glacial | |
Serio | Serio | Italy | 166 | Glacial | |
Multnomah | Multnomah Creek | Oregon, USA | 165 | Missoula floods hanging valley | Basalt |
Various schemes have been developed to classify waterfalls both in terms of their characteristics and their origins. An excellent early model on origins was provided by Lobeck (1939: 136) and this is reproduced as Fig. 1.
Waterfalls occur in almost all climatic environments. In terms of negative factors, they are scarce in many arid areas because of a lack of stream flow; but even here, extreme rainfall events and past pluvial episodes may explain the existence of
In terms of positive factors, they are common in formerly glaciated areas such as Norway, the European Alps and the South Island of New Zealand because of the creation of hanging valleys by glacial erosion or because of glacial diversion of drainage. It was also postulated by Birot (1968), Büdel (1982) and Tricart (1965) that they and cataracts were common in humid tropical areas because of the fact that deep weathering meant that there was a shortage of coarse sediment in stream courses to cause removal of irregularities. Subsequent research has thrown doubt upon the generality of this supposition (Ollier 1983).
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
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,
The caprock model is not of universal applicability as pointed out by Young (1985). Young et al. (2009: 202–204) wrote:
In this section, the links between rock type and water fall development are explored in the context of specific regional sites.
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:
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).
Waterfalls occur in a wide range of geomorphological settings, as indicated in Lobeck’s diagram in Fig. 1. A similar list of settings was also provided by Buckle (1978: 110–111) who suggested that the following are the main causes of waterfalls:
an outcrop of hard rock overlying softer rocks in the river bed, faulting across the river bed, where the river enters the sea at a cliff line following erosion or where sea level has fallen, where a tributary hanging valley enters a glacially over-steepened major valley, a lava or landslide may create a lake and a waterfall that occurs where overspill drops over the edge of the barrier, where a river falls over a plateau edge, where rejuvenation of a river valley has formed a sharp knickpoint.
Waterfalls can also occur where tectonic uplift of the entire river network is too rapid for smaller streams to respond, so tributaries can become very steep or have convex longitudinal profiles, as well as a waterfall at the main channel junction.
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
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,
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).
In the original caprock model, Lyell (1875) mentioned the role of both spray and frost weathering of shale in causing waterfall recession. Bishop and Goldrick (1992) believed that failure at joint planes along the lip of falls seems to be the major cause of retreat in two examples from New South Wales. They noted that potholes, perhaps caused by cavitation, drill down from above.
Haviv et al. (2006, 2010) also stressed the role of gravitational failures, direct abrasion by the falling jet, direct abrasion by plunge pool rollers, wet–dry weathering, frost attack and seepage weathering. Similarly, Hayakawa (2013), working in Japan, recognised the importance of multiple processes accounting for retreat, including rockfalls, surface water free fall load, freeze–thaw or wet–dry weathering and cavitation at the lip of the falls. Cavitation may indeed be an underestimated cause of bedrock erosion (Whipple et al. 2000). Lamb and Dietrich (2009) argued that although many people have proposed that waterfalls retreat by undercutting, many propagating waterfalls maintain a vertical face in the absence of undercutting. They stressed that vertical waterfalls can remain vertical in retreat due to toppling in bedrock with near-horizontal and vertical sets of joints. At a waterfall, faces are affected by shear and drag from the overflowing water, buoyancy from the plunge pool at the base of the waterfall and gravity. They also suggested that although seepage erosion has been proposed as an alternative to plunge pool erosion, the evidence for seepage flow is ambiguous and cannot generally explain the excavation of coarse collapsed debris. Likewise, Lamb et al. (2007) believed that in Hawaii, surface runoff rather than seepage carves amphitheatrical headed valleys. Plunge pool erosion by powerful streams with relatively large catchments and their sediment load leads to steep waterfalls, though jets of high-velocity water can cause clear water erosion (Pasternack et al. 2007). Lapotre and Lamb (2015) also believed that flow acceleration means that flow erosion is accelerated at the brink of a waterfall and thus promotes plucking and toppling of jointed rock. Plunge pools, the character of which is affected by factors such as flow velocities, sediment supply and grain sizes, are significant components of waterfall systems (Elston 1917, Scheingross, Lamb 2016), and the work of Scheingross, Lamb (2017) pointed to the importance of vertical drilling of successive plunge pools for propagation of upstream migration rather than the undercutting model. However, Scheingross et al. (2019: 229) proposed that
Fig. 7 shows a large plunge pool at the base of a dry waterfall formed in quartzite at Etusis, central Namibia. Pothole erosion is also an important process at the Augrabies Falls (Springer et al. 2006). There is now a large literature on the factors affecting the development of plunge pools beneath artificial dams, and this may provide insights into the development of natural plunge pools (e.g. Melo et al. 2006, Fiorotto et al. 2016).
Not all waterfalls will necessarily undergo recession at any appreciable rate. As Lake (1925: 249) stated:
Furthermore, rates of waterfall recession will vary in time. For example, retreat may be rapid after a fault causes a fall to develop across a watercourse. This was the case in Taiwan (Hayakawa et al. 2010). Surface ruptures along the Chelungpu thrust fault caused formation of waterfalls when the 921 Chi-Chi Earthquake occurred on September 21, 1999. Since then, they have receded upstream at extremely rapid rates, causing bedrock incision for tens to hundreds of metres in length within a decade. Field measurements revealed that the mean rate of a knickpoint recession in the largest river (Ta-chia) was 3300 mm per year in the earlier 6 years (1999–2005) and 220,000 mm per year in the last 4 years (2005–2009). This acceleration of the recession may have been due to an increase in flood frequency and intensity, narrowing of the channel width and/or anisotropy of rock strength sandstones and mudstones along the stream. The other knickpoints in the area showed relatively similar recession rates throughout the decade on the order of 20,000–60,000 mm per year.
Another cause of changes in rates of recession through time is the changes in stream discharge and abrasive sediment transport. For example, depletion of paraglacial sediment supply during the Holocene can lead to a deficiency in tools for bedrock erosion.
Dating of landforms and archaeological sites enables estimates to be made of the long-term rates of recession. In the case of the Victoria Falls, Moore and Cotterill (2010) estimated that the rate of recession up the Batoka Gorge was between 42 and 80 mm per year, implying that headward erosion has incised 20 km of gorges below the falls in c. 300–250 ka. Derricourt (1976), also working on the Victoria Falls, used archaeological data to estimate the rates of retreat and suggested a rate of 150 mm year over 20,000 years.
Berlin and Anderson (2007) showed that Late Cenozoic incision of the Colorado River led to isolation of the Roan Plateau in SW USA. This initiated knickpoints and a wave of erosion, with the knickpoint recession rate being a function of drainage area and rock susceptibility to erosion. Knickpoint recession speeds declined through time as catchments became smaller – they started at c. 7.1–11.9 mm per year and have now dropped to 0.3–2.3 mm per year.
Other long term rates have been estimated for glaciated valleys (Hayakawa 2011). The author found that recession rate since deglaciation depends on the erosional power of streams and bedrock resistance. Examples were given from Yosemite and the Swiss Alps. For Lower Yosemite (100 m high), the rate was 46 mm per year; for Isola (60 m high), the rate was 55 mm per year; and for Sils (50 m high), the rate was 75 mm per year. Hayakawa and Wohl (2006) studied the 12-m-high Poudre Falls of the Rocky Mountains Front Range in Colorado. Developed in granite, they had recessed by over 1000 m (90 mm per year) over the 12,000 years since glaciers had retreated from the valley. Sardeson (1908) studied the St Anthony Falls on the Mississippi River and estimated a post-glacial rate of recession of c. 744 mm per year.
In the basalts of the Golan Heights of Israel, the back erosion of the Sa’ar river falls over a period of 100,000 years was 0.68 mm per year (0.68 km) (Shtober-Zisu et al. 2018). In the volcanic terrains of Japan, Hayakawa et al. (2008a) found that the rate of recession, based on the age of ignimbrites, was 13–68 mm per year for the Aso volcanic area, where the height of falls was 8.3–63.3 m. They found that the recession rate depends on discharge, width and height of the fall and on the rock strength (both of which can be estimated). Hayakawa et al. (2008b) studied the 322-m-high Shomyo Falls of central Japan, which had formed in pyroclastic materials of known age. Their estimated rate over 100,000 years was 80–150 mm per year, whereas the current modelled rate using the force/resistance (F/R) index (described below) was only 6–11 mm per year. They believed that this may be due to reduced post-glacial sediment loadings and flow. Hayakawa (2013) estimated the retreat rate for the 98-m-high Kegon Falls, which had developed in andesitic lava since c. 20,000 BP, was 18 mm per year. He noted, however, that a single large rockfall in 1986 led to a recession of c. 8 m. Hayakawa and Matsukura (2003) examined the Beso Falls in Japan. Developed in mud-stones, and with heights of 1.8–32 m, rates ranged from 13 to 270 mm per year. Yoshida et al. (2017) studied waterfalls in southern Kyushu, which had developed in ignimbrites100 ka old. The estimated recession rates for six falls (c. 20 m high) were 2–30 mm per year. Mackey et al. (2014) used cosmogenic dating of the Ka’ula’ula waterfall on Hawaii. This has migrated backwards at a rate of 33 mm per year over the past 120 ka.
The rate of retreat of the Niagara Falls has been studied for a long time (Philbrick 1970, 1974, Tinkler 1987, Tinkler et al. 1994, Pryce 1995). Gilbert (1895) calculated a Holocene recession rate of 4–5 feet (c. 1200–1500 mm) per year. Hayakawa and Matsukura (2009) found that the rate of retreat had declined from c. 1000 mm per year a century ago to c. 100 mm year at present. This was due partly to water abstraction by humans, and partly to a natural increase in waterfall lip length. Stevaux and Latrubesse (2010) estimated that the great Iguazu Falls have retreated upstream at a rate calculated to be c. 14–21 mm per year (21–42 km) over the last 1.5–2.0 million years.
The above data are summarised in Table 3. There is a great spread of values, but a characteristic rate appears to be a few tens of millimetres per year.
Rates of waterfall recession ordered according to the rate in millimetres per year.
Location | Source | Rate |
---|---|---|
Golan Heights, Israel | Shtober-Zisu et al. (2018) | 0.68 |
Roan Plateau | Berlin, Anderson (2007) | 0.3–11.9 |
Kyushu, Japan | Yoshida (2017) | 2–30 |
Aso, Japan | Hayakawa et al. (2008a) | 13–68 |
Iguazu | Stevaux, Latrubesse (2010) | 14–21 |
Beso, Japan | Hayakawa, Matsukura (2003) | 13–270 |
Kegon, Japan | Hayakawa (2013) | 18 |
Hawaii | Mackey et al. (2015) | 33 |
Victoria Falls | Moore, Cotterill (2010) | 42–80 |
Yosemite | Hayakawa (2011) | 46 |
European Alps | Hayakawa (2011) | 55–75 |
Shomyo, Japan | Hayakawa et al. (2008b) | 80–150 |
Poudre Falls, USA | Hayakawa, Wohl (2006) | 90 |
Victoria Falls | Derricourt (1976) | 150 |
Niagara | Hayakawa, Matsukura (2003) | 100–1000 |
St Anthony Falls, USA | Sardeson (1908) | 744 |
Niagara | Gilbert (1895) | 1200–1500 |
Taiwan | Hayakawa et al. (2010) | 3300–220,000 |
Hayakawa and Matsukura (2003) found in their study of the Beso Falls of Japan, there was a good correlation of recession rates determined from the landform ages with an F/R index. This is based on annual precipitation, width and height of fall, water density and rock strength obtained by Schmidt hammer:
DiBiase et al. (2015) argued that the primary controls on waterfall retreat rates are rock strength, joint orientation, coarse sediment supply and water discharge. Coarse sediment abrades waterfall lips, drills plunge pools and erodes non–waterfall-intervening stretches.
Shelef et al. (2018) were of the opinion that waterfall recession rates are controlled by a large range of factors, including plunge pool drilling, freeze–thaw and wet–dry cycles and groundwater seepage. The intensity of these processes depends on factors such as caprock and sub-caprock strengths, joint density, sediment concentration and grain size distribution, water discharge, the micro-topography of the waterfall lip, waterfall height, water jet impact angle and the properties of the lag debris.
Some studies have found a correlation between drainage area and recession rates (Crosby, Whipple 2006), but this is not universally the case (Mackey et al. 2014, Baynes et al. 2018).
Although the greatest interest has been in the rates of waterfall recession, there are examples of waterfalls that prograde, such as those on the Dunn’s River in Jamaica. These are the waterfalls which were described by Gregory (1911) as
Subsequently, constructive waterfalls have been recorded from many areas, both wet and dry, including the Naukluft National Park in Namibia (Viles et al. 2007, Goudie, Viles 2015), where at Blasskranz, one tufa cascade is some 80 m high and 400 m wide (Fig. 8). Other studies of tufa waterfalls include those of Dramis and Fubelli (2015) in Ethiopia, Wright (2000) in the Kimberley District of northwestern Australia, Zhang et al. (2001) in China, Pawar et al. (1988) in India, Marker (1971) in South Africa, Sanders et al. (2006) in the European Alps, Donovan et al. (1988) in Oklahoma, USA, Ray and Rahn (1977) in South Dakota, USA, Harbor et al. (2005) in the Central Applachians, USA, Travassos et al. (2016) in Brazil, Bonacci et al. (2017) in Croatia, Ukey and Pardashi (2019) in the Deccan of India and Edgell (2006) in southern Oman (e.g. Wadi Darbat). Small waterfalls may also be associated with silica-depositing springs, as in New Zealand (Migoń, Pijet-Migoń 2016).
Waterfalls are both numerous and widespread, and they occur on a large range of rock types and under many different climatic and geomorphic conditions. They are moulded by a range of processes, including the undercutting of a caprock, plunge pool incision, toppling and various types of weathering. Many estimates have now been made of their rates of recession over different timescales, though some waterfalls – constructive waterfalls – may be characterised by aggradation and progradation. Waterfalls are geomorphologically important because,