A waterfall is a sudden break in a river's slope. Geologists call that break a knickpoint, and rivers spend their whole lives trying to erase them — grinding the lip down, cutting the step back upstream, smoothing the profile. The waterfalls we travel to see are the ones where something is winning that fight, at least for a few thousand years.
There are only a handful of ways a river ends up falling off an edge. Once you know them, you can read most waterfalls at a glance.
1. Hard rock over soft rock
The classic mechanism. A resistant layer — a caprock of dolostone, sandstone or basalt — sits over something softer, like shale. The river cuts through the soft layer faster, undermining the hard one until slabs of caprock break away. The waterfall retreats upstream, one collapse at a time, and leaves a gorge behind it.
Niagara is the textbook case. Hard Lockport dolostone caps softer shales, and since the ice retreated at the end of the last glaciation the falls have cut back roughly 11 kilometres from the Niagara Escarpment, leaving the Niagara Gorge as the record. Horseshoe Falls was historically retreating by around a metre a year; diverting water for hydropower and reshaping the crest has since slowed it dramatically. High Force in England does the same thing over a band of hard dolerite called the Whin Sill.
2. Cracks in a lava plateau
Victoria Falls looks like a single cliff, but it is a crack. The Zambezi flows across a plateau of ancient basalt laced with fractures filled with softer sediment. Where the river finds one of those weaknesses running across its path, it scours it out into a slot and pours in. The zigzag gorges below the falls mark earlier positions — each one a crack the river opened, then abandoned when it found the next one upstream. That is why you can stand on the far lip of the chasm, a stone's throw from the falling water, at the same height as the top.
Basalt shows up again at Iguazú, where the river spills off the edge of a vast lava plateau in hundreds of separate channels, and in Iceland, where Svartifoss drops past hexagonal basalt columns formed as lava cooled and shrank.
3. Valleys the glaciers left hanging
Ice is a far more powerful excavator than water. During the ice ages, big trunk glaciers gouged their valleys much deeper than the small tributary glaciers feeding them. When the ice melted, the side valleys were left perched high on the walls of the main one — hanging valleys — and their streams now fall hundreds of metres to reach the floor.
This is the origin of most of the world's tallest free-falling water: Yosemite Falls and Bridalveil in California, the Seven Sisters and Mardalsfossen in Norway, Stirling and Sutherland in New Zealand's Fiordland, and Staubbach in Switzerland's Lauterbrunnen valley. Hanging-valley falls are often horsetails or plunges, and they tend to be at their fullest during snowmelt rather than the rainy season.
4. Escarpments, faults and tepuis
Sometimes the step comes first and the river simply finds it. A fault can lift one block of land above another; a long escarpment can mark the edge of a plateau. Tugela Falls tumbles off the Drakensberg's Amphitheatre escarpment in a series of drops. Angel Falls, the tallest uninterrupted waterfall on earth, pours off the rim of Auyán-tepui — a flat-topped sandstone mesa in Venezuela that rises sheer out of the rainforest. Kaieteur in Guyana drops off the edge of the same ancient sandstone formation.
A former coastline works too. Skógafoss in Iceland falls over what used to be sea cliffs; the shore has since moved several kilometres away.
5. Waterfalls that build themselves
Most waterfalls are temporary: they are eating their way upstream. A few do the opposite. Where river water is saturated with dissolved limestone, calcium carbonate precipitates out as tufa or travertine — encouraged by mosses and algae that trap it as they grow. Over centuries the deposits build dams, and the dams build waterfalls.
Plitvice in Croatia is a staircase of sixteen lakes held back by tufa barriers, and the barriers are still growing. Krka, Kuang Si in Laos, Erawan in Thailand, Agua Azul in Mexico and Havasu in Arizona all owe their terraces — and their startling blue-green water — to the same chemistry. It is also why swimming is banned at several of them: the formations are fragile, and they are the waterfall.
6. Waterfalls underground
Limestone dissolves, and water finds the gaps. Ruby Falls in Tennessee drops inside a cave far below Lookout Mountain. In Switzerland, Trümmelbach carries glacial meltwater through a corkscrew of chutes carved inside the mountain, reached by a tunnel lift. In Iceland, Hraunfossar emerges sideways out of a lava field as hundreds of small springs.
The pool at the bottom
Falling water hits hard, and it carries rock with it. The churning at the base drills a plunge pool, often deeper than the waterfall is high in soft rock, and the recirculating current that does the drilling is also what makes the base of a big fall dangerous to swim in. More on that in our safety guide.
Why this matters for a trip
Geology decides the season. Hanging-valley falls peak with snowmelt and can dry to a thread by late summer. Plateau and rainforest falls follow the rains, often months after they fall upstream. Spring-fed and lake-regulated falls barely change. Before you pick dates, check the month strip on each destination page — or start from the month you can travel.
For the shapes waterfalls take once they form, see the eight types of waterfall. For the vocabulary, the glossary.
