How Waterfalls Are Formed
There is arguably no better starting point for a discussion of the geological origins of waterfalls than to examine the genesis of one of the most thunderous of them all, Niagara Falls. Fed by the Great Lakes, the world's largest fresh water system, this behemoth pours out at daytime peak hours a volume of six million cubic feet of water per minute, and dates back some 18,000 years to the last Ice Age.
Each waterfall has its own unique set of origins details. In the case of Niagara Falls, a series of shifts in the landscape from 18,000 to 5,500 years ago culminated in a relatively short but momentous event. The fully re-engorged waters of the Niagara River unearthed a riverbed that had been buried beneath the surface, causing a 90-degree turn in the flow of the water and accelerating a formative process at the base known as cavitation. The air escaping from water as it falls creates cavities within the liquid, which then collapse at the bottom of the waterfall and send waves of disintegrating power to the nearby rocks.
Glaciers and Volcanic Activity
Next to Niagara Falls, the most popular waterfall attraction in North America is the series of cascading streams and rivers in Yosemite National Park. These include Yosemite Falls, Bridalveil Fall, Horsetail Fall and Wapama Falls. Like the Niagara River, the most recent Ice Age was the main culprit. Because a number of smaller glaciers wedged onto the main, large ice formation that once covered Yosemite, a number of small valleys were carved high up on the sides of the valley and turned into waterfalls when the water melted. In the case of Hawaii, the formation of waterfalls has more to do with volcanic activity than glaciers.
Gorges and Valleys
But it is on the continent of Africa that formed what is generally considered today to be the most spectacular waterfall on the planet. Zimbabwe's Victoria Falls is the result of the raising of the Makgadikgadi Pan area roughly two million years ago. Ever since that time, the Zambezi River has been flowing into the basalt and finding its way to the weaker points. The resulting gorges, some of which now rest below the present main falls, are what combined to create a true wonder of the world.