How Are Waterfalls Formed? Seven Settings and One Exception

A waterfall needs a river and a step in its bed. Where the step comes from is where the explanations split. The story most people learn, a layer of hard rock sitting on soft rock, is true for Niagara and does not describe Victoria Falls or Iguazú. In his 2020 review of waterfall science in Quaestiones Geographicae, the Oxford geographer Andrew Goudie says the caprock model does not apply to those two, which sit on thick sequences of basalt. This page goes through the settings that do produce steps, one at a time, with a named example and the paper or agency that says so. We read every source named here on 2026-10-27.

Some links on this site lead to tour listings. We earn a commission on bookings made through them, at no extra cost to you. See our Editorial Policy. Nothing below depends on a booking. The places we name are there so you can stand in front of each setting, and our page for Niagara Falls is the nearest one to the textbook case.

A textbook explanation fits one rock sequence. Before you repeat it for the next waterfall, find out what the rock is.

Victoria Falls is a different story again. The IUGS page describes the falls as the present position of a knickpoint, a sudden steepening of the river bed, that has migrated west and cut the gorges below into layered Jurassic basalt. It says the Zambezi runs in a deep graben and points to tectonic control (IUGS). The UNESCO text adds the pattern that makes aerial photos of the site unmistakable: the gorges take a zigzag course for about 150 km below the falls, and seven previous waterfalls occupied the seven gorges below the present one (UNESCO, read through an archived copy). Both texts speak of eight gorges in all. Goudie adds river capture. A possible explanation, he writes, is that the Upper Zambezi was captured by a tributary of the middle Zambezi in the late Cenozoic, and the lowered base level then drove fast cutting back toward the falls. Our page for Victoria Falls lists the tours.

Goudie's review sorts waterfalls by geomorphological setting, and we group his sections into seven, counting the caprock case and the basalt cases above as the first two. The ground itself can step. He calls Kalambo Falls, on the border of Zambia and Tanzania, the classic case of a rift valley changing the base level, and he places Murchison Falls on the Nile where the river plunges over schists at the Western Rift. Southern Africa shows the other form, great escarpments at the edge of the continent, with notable falls on the Kunene at Ruacana, the Orange at Augrabies and the Tugela. A river that meets the sea at a cliff drops over it, and a falling sea level does the same. Goudie's example is Ka'ula'ula in Hawaii, a waterfall that has migrated back over the past 120,000 years from a knickpoint started by sea-cliff erosion at a time of high sea level.

Ice leaves waterfalls behind in whole regions. A glacier deepens the main valley faster than its tributaries can follow, and when it melts the side streams are left hanging. The University of Oregon's geology page describes it that way, with Bridal Veil Fall in Yosemite as its picture. Goudie says waterfalls are widespread in areas glaciated in the Pleistocene, such as the fjords of Norway and New Zealand and the flanks of Yosemite. He also lists Multnomah Falls in Oregon in his table of tall falls, classing it as a hanging valley shaped by the Missoula floods, which brings us to the last two settings. A lava flow or a landslide can block a river and make a lake that spills over the face of the dam; Goudie gives the Kegon Falls of Japan as a lava-dam case. And a catastrophic flood can cut a steep, blunt head into a canyon. He cites Palouse Falls in Washington State, made by the glacial Lake Missoula floods, and Malham Cove in Yorkshire, a dry cliff that ran as a waterfall for the first time in living memory in the very wet winter of 2015 and 2016. The Columbia River Gorge page lists tours in that country.

The falling water digs it. Gilbert's account of Niagara is the clearest: the blocks that fall from the cornice are rolled by the plunging water and grind out a hollow, and as the cataract retreats it leaves a deep channel behind it where the current is slow. Goudie's review adds the modern list of processes that make a fall retreat: toppling of jointed rock, flow that speeds up at the brink, cavitation at the lip, and scour by a pool that is itself shaped by the sediment it carries. Pools are not universal. He quotes a study by Young and colleagues in which many waterfalls in sandstone turned out to be buttressed at the base instead of undercut, and some had no plunge pool at all. For a visitor the point is plain. A pool at the foot of a fall is deep for reasons that have nothing to do with swimming, and what each park allows is in Can You Swim at a Waterfall?, with the hazards at the top in Waterfall Safety: The Rules That Matter.

Yes, and it is the exception that gets its own name. A tufa waterfall is built by the water that falls over it. The Plitvice Lakes National Park describes tufa as a hollow, porous rock formed when dissolved calcium carbonate is deposited by plants, algae and mosses, and it lists the conditions: water oversaturated with calcium carbonate, a pH above 8.0, and dissolved organic matter below 10 milligrams per liter. Goudie credits the term constructive waterfalls to J. W. Gregory in 1911, whose examples came from the limestone regions of the Balkans: the Krka falls in Dalmatia and the Pliva and Topolje falls in Bosnia. Goudie also names Dunn's River in Jamaica among the waterfalls that prograde. Those falls advance. The word for the rock changes with the source, which the glossary sorts out, and growth rates and the rules on a barrier are in Tufa and Travertine Waterfalls. Tours to the two Croatian parks are listed on our Plitvice Lakes and Krka National Park pages.

For the caprock case, Niagara Falls. For the basalt giants, Iguazú Falls and Victoria Falls. For tufa, the two Croatian parks above. Whether a tour is worth booking is a separate question, and our guide to reading a tour listing is where to start. The rest of this section follows the same rule as this page: each claim names its source and the day we read it.