Surface water is water that is found on the Earth's surface, such as in rivers, lakes, streams, and reservoirs. Of all the types of surface water, lakes contain the greatest amount of freshwater. Lakes are formed when water collects in a depression on the land surface, and they can range in size from small ponds to large bodies of water like the Great Lakes. Lakes are a valuable source of freshwater, and they are often used for drinking water, irrigation, and recreation.