Land Run-off or Surface runoff is water, from rain, snow melt, or other sources, that flows over the land surface, and is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called overland flow. A land area which produces runoff draining to a common point is called a watershed. 

Rain water which falls on land run to the sea. Often they are full of pollutants that wreck the ocean. There are “dead zones,” places in the sea where nothing can live because chemicals promote the growth of plants that suck up oxygen or the “garbage patch” where plastic from around the world floats to the north Pacific and becomes nature’s giant waste dump.