Surface runoff is the water that flows over the land surface due to precipitation, snowmelt or other sources. This water ultimately reaches rivers, lakes, and oceans. There are a variety of synonyms for the word surface runoff that can be used interchangeably. Some of these synonyms include overland flow, sheet flow, non-point source pollution, stormwater runoff, and urban runoff. Overland flow refers to water that moves over the land surface without being confined to a channel. Sheet flow refers to water that flows over a surface in a thin sheet. Non-point source pollution refers to pollution that originates from diffuse sources such as agricultural, urban and suburban land use. Stormwater runoff and urban runoff specifically refer to surface runoff from areas of high population density, such as towns and cities.