<P> Often, the rock stratum just below the more resistant shelf will be of a softer type, meaning that undercutting due to splashback will occur here to form a shallow cave - like formation known as a rock shelter under and behind the waterfall . Eventually, the outcropping, more resistant cap rock will collapse under pressure to add blocks of rock to the base of the waterfall . These blocks of rock are then broken down into smaller boulders by attrition as they collide with each other, and they also erode the base of the waterfall by abrasion, creating a deep plunge pool in the gorge downstream . </P> <P> Streams can become wider and shallower just above waterfalls due to flowing over the rock shelf, and there is usually a deep area just below the waterfall because of the kinetic energy of the water hitting the bottom . However, a study of waterfalls systematics reported that waterfalls can be wider or narrower above or below a falls, so almost anything is possible given the right geological and hydrological setting . Waterfalls normally form in a rocky area due to erosion . After a long period of being fully formed, the water falling off the ledge will retreat, causing a horizontal pit parallel to the waterfall wall . Eventually, as the pit grows deeper, the waterfall collapses to be replaced by a steeply sloping stretch of river bed . In addition to gradual processes such as erosion, earth movement caused by earthquakes or landslides or volcanoes can cause a differential in land heights which interfere with the natural course of a water flow, and result in waterfalls . </P> <P> A river sometimes flows over a large step in the rocks that may have been formed by a fault line . Waterfalls can occur along the edge of a glacial trough, where a stream or river flowing into a glacier continues to flow into a valley after the glacier has receded or melted . The large waterfalls in Yosemite Valley are examples of this phenomenon, which is referred to as a hanging valley . Another reason hanging valleys may form is where two rivers join and one is flowing faster than the other . Waterfalls can be grouped into ten broad classes based on the average volume of water present on the fall (which depends on both the waterfall's average flow and its height) using a logarithmic scale . Class 10 waterfalls include Niagara Falls, Paulo Afonso Falls and Khone Falls . </P> <P> Classes of other well - known waterfalls include Victoria Falls and Kaieteur Falls (Class 9); Rhine Falls and Gullfoss (Class 8); Angel Falls and Dettifoss (Class 7); Yosemite Falls, Lower Yellowstone Falls and Umphang Thee Lor Sue Waterfall (Class 6); Sutherland Falls (Class 5). </P>

Where does the water come from for yosemite falls