Waterfalls are places where rivers fall vertically. Waterfalls may also occur at places on rivers where the water passes over a hard rock joined to a softer rock. The river does not erode the hard rock as much as a softer rock and so the harder rock forms a shelf over which the river flows. The world's highest falls are the Angel Falls in Venezuela, which have a total drop of 979 meters. Angel Falls are named after an American pilot Jimmy Angel who flew over them in 1935. The Victoria Falls in Zimbabwe fall on River Zambezi in Africa. They have a total drop of 500 meters. The roar from the Victoria Falls can be heard as far as 40 km. In the rainy season, water flows at the rate of 550 million liters per minute. Another famous waterfall is the Niagara Falls on the US-Canadian border. These falls were first formed about 12,000 years ago. Since then, the falls have moved 11km further upriver. ||