Wherever depressions occur on land surfaces, water will accumulate and flow
Surface runoff is the flow of water that occurs when excess stormwater flows over a surface. This occurs when soil is saturated to full capacity, because rain arrives more quickly than soil can absorb it, or because imperviuos areas ( roads and pavements) send their runoff to surrounding soil that cannot absorb all of it. Surface runoff is a major component of the water cycle. It is the primary agent in soil erosion by water.
A land area which produces runoff that drains to a common point is called a drainage basin such as this one. This is an example of a well designed and well constructed runoff channel leading to a drainage basin. This one has a marshy swamp area in the center of the drainage basin. During very heavy storms, this basin has filled to form a small lake, but within a day or two, the water dissapates completely and the marsh, and the birdlife that resides here does not seem to be affected at all.
In addition to causing water erosion and pollution, surface runoff in urban areas is a primary cause of urban flooding, and street flooding.
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