A waterfall is a natural feature where water cascades or falls vertically or nearly vertically over a cliff or steep slope, often as part of a river or stream. It occurs when the flow of water encounters a sudden drop in elevation, such as a cliff or ledge. Waterfalls can vary in size, from small trickles to massive falls, and are often found in mountainous or hilly terrain. They can be a single drop or a series of cascades, and are often popular scenic attractions.