Waterfalls usually occur in young rivers, which haven’t had enough time to wear down all the land in its path.
Rushing water erodes softer materials before harder ones. Where the terrain includes both soft and hard materials, the water carries away the soft material, but must fall over the harder material.
Cliffs of hard rock take the longest time to erode, so water winds up cascading—sometimes great lengths—over them to continue its path.
At 3,281 feet (1,000 meters), Angel Falls in Venezuela holds the record as the highest waterfall in the world.
Niagara Falls, between Canada and the United States, is only 167 feet (51 meters) high. But these dramatic falls stretch over 1,060 feet (323 meters) of land and 1.25 million gallons (5.6 million liters) of water rushes over them every second.
As with all waterfalls, Niagara Falls is moving upstream as the water continues to erode the land.