A hydrologist studies water systems in order to solve problems related to water quality and quantity. They study the way that rain, snow, and other forms of precipitation move across the land to bodies of surface water such as lakes and streams or the way precipitation moves through soil and rocks to underground water below the earth’s surface. Hydrologists analyze how changes to the environment influence water and how water changes the surrounding environment. A hydrologist may study water in order to control river flooding, to find water supplies for cities or farms, or to prevent or clean up pollution caused by contamination.