A lake (Old English lacu) is a terrain feature (or physical feature), a body of liquid on the surface of a world that is localized to the bottom of basin (another type of landform or terrain feature; that is not global). Another definition is a body of fresh or salt water of considerable size that is surrounded by land. On Earth a body of water is considered a lake when it is inland, not part of the ocean, is larger and deeper than a pond.[2][3] The only world other than Earth known to harbor lakes is Titan, Saturn's largest moon, which has lakes of ethane, most likely mixed with methane. It is not known if Titan's lakes are fed by rivers; Titan's surface is carved by numerous river beds. There is evidence that Mars once held lakes of water.
A waterfall is a place where flowing water rapidly drops in elevation as it flows over a steep region or a cliff. Waterfalls are considered to be one of the most beautiful phenomena in nature.[1] Some waterfalls are used to generate hydro-electric power.
