Surface runoff occurs when the amount of rainwater falling on the surface of the ground is greater than the infiltration rate (rate at which the water filters into) of the surface.

The water flows over the surface of the land to points of lower elevation, evetually reaching streams, rivers, lakes and on to the oceans.

Globally, runoff occurs because of the imbalance between evaporation and precipitation over the Earth's land and ocean surfaces (see: The Water Cycle). It invloves a number of events; The intensity of rainfall exceeds the infiltration rate. The formation of a thin water layer on the ground which begins to move under the influnce of and gravity and the slope of the land. The flowing water accumulates in depressions in the ground. The depressions overflow forming small rills. The rills merge to form streams and rivers. Streams and rivers empty into lakes and the oceans.

The water moving across the surface of the land picks up soil particles and other debris which is responsible for the discolouration of many rivers which receive a significant proportion of their water as runoff (c.f.: chalk streams).

As well as such natural impurities, surface runoff collects various pollutants present on the land from human activities such as fertilizers and pesticides from farmland and a wide variety of materials from land contaminated by industry. All of these cause considerable problems when the runoff reaches watercourses.

During periods of torrential rain when the soil is waterlogged, surface runoff can cause flash floods as the water runs down steep-sided valleys causing much devastation and loss of life as at Boscastle in Cornwall (2004) and Lynmouth in Devon (1952).

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