Run-off

Run-off is rainwater (or melted snow) that flows over the surface of the land rather than soaking into the ground. It is one of the most important stages of the water cycle: the way most fresh water eventually ends up in rivers, lakes and the sea. Run-off shapes the landscape (cutting valleys and depositing soil), feeds entire ecosystems, and provides the water we drink. But too much run-off too quickly can also cause floods, soil erosion and pollution.

  • What it isSurface water flowAfter rain or snowmelt
  • Eventually reachesRivers and the seaOr evaporates on the way
  • % of rain that runs offVaries hugelyFrom under 5% (forests) to over 70% (cities)
  • Worst causes of floodingConcrete, heavy rain, deforestation
  • CarriesSediment, nutrients, pollutionOften into rivers
  • Best slowdownTrees, wetlands, healthy soil

What happens to rain when it lands

When raindrops hit the ground, several things can happen.

  • Soak into the soil: most rain on healthy soil sinks down into the ground, where it can later become groundwater or be taken up by plants.
  • Evaporate: especially on hot dry surfaces.
  • Run off the surface: when the rain is heavier than the soil can absorb, or where the ground is too hard or too steep, water flows downhill across the land.
  • Be intercepted by plants: trees and other plants catch some rain on their leaves before it reaches the ground.

The balance between these depends on the rainfall, the soil and the land cover.

What affects how much runs off

  • Rainfall intensity: a gentle drizzle mostly soaks in; a sudden downpour mostly runs off.
  • Soil type: sandy soil drains fast and absorbs lots; clay soil drains slowly and produces more run-off.
  • Slope: steeper land produces faster, heavier run-off.
  • Plant cover: forests and grasslands slow water down and let it soak in. Bare ground produces more run-off.
  • Land use: concrete, tarmac and roof tiles cannot absorb water at all. Cities produce far more run-off than countryside.
  • Existing soil moisture: dry soil can absorb a lot of rain; already-saturated soil produces immediate run-off.

How run-off shapes the land

Over thousands and millions of years, run-off has carved the landscape. Tiny rivulets join to form streams, streams join into rivers, rivers cut valleys into the rock. Famous landforms shaped by run-off include:

  • The Grand Canyon, cut by the Colorado River over 5 to 6 million years.
  • Almost every river valley in the UK.
  • Gullies: small steep-sided channels cut by intense run-off, often on farmland.
  • Deltas: triangular fans of sediment where rivers slow down and dump their load (like the Nile Delta or the Mississippi Delta).
Fact Concrete, tarmac and roofs in a typical city mean that approximately 55% of rainfall in an urban area runs straight off into drains rather than soaking into the ground. In a forest, that figure is usually under 5%. That difference is one of the main reasons cities are so prone to flash flooding: rain that would normally have taken hours to reach a river now reaches it in minutes.

Why run-off can be a problem

  • Flooding: when too much water reaches a river too quickly, the river can burst its banks.
  • Soil erosion: fast-moving water carries soil with it. A single heavy storm can erode a year's worth of topsoil from bare fields.
  • Pollution: run-off picks up oil from roads, fertilisers from farms, pesticides, plastic and animal waste, then dumps the lot into rivers and the sea.
  • Algal blooms: fertilisers washed into water bodies cause sudden explosions of algae that kill fish and other wildlife.

How to slow run-off down

Several techniques can reduce the harmful effects of run-off.

  • Plant trees: forest cover dramatically reduces run-off, both by intercepting rain and by improving soil structure.
  • Restore wetlands: bogs and marshes act like giant sponges, absorbing water and releasing it slowly.
  • Build sustainable urban drainage: ponds, permeable paving and green roofs all help cities absorb rain.
  • Use cover crops: keep farmland under plant cover all year instead of bare ploughed soil.
  • Build buffer strips along rivers: strips of grass or trees between fields and water bodies catch run-off before it pollutes the river.
Did you know? Many cities now build sustainable drainage systems (or SuDS) that copy how nature handles rainfall. Instead of channelling water straight into pipes, they use ponds, swales (shallow grassy ditches) and rain gardens to slow water down and let it soak in. Sheffield, Manchester and other UK cities have built networks of SuDS that significantly reduce flooding.
Deeper dive: how Beavers are slowing flooding in British rivers

One of the most exciting developments in flood management has been bringing back the European beaver, which was hunted to extinction in Britain around 400 years ago. Trial beaver releases have been running in Devon, Scotland, Cornwall and several other places since the early 2010s, and the results have been impressive.

Beavers are nature's engineers. They build dams across streams using sticks, mud and stones, creating ponds upstream. The ponds slow water down, store huge volumes during heavy rain, and release water gradually during dry periods. A single beaver family's dams can reduce peak flood flow downstream by 30% or more, while also raising the water table and creating new wetland habitat for fish, birds and other wildlife.

One of the best-studied sites is the River Otter in Devon, where beavers released in 2008 produced measurable reductions in flooding downstream during major storms in 2014 and 2019. Their dams hold back tens of thousands of litres of water at a time, doing for free what concrete flood barriers would cost millions to build.

The UK government formally recognised beavers as a native species again in 2022, and there are now plans to gradually allow more beaver reintroductions in suitable rivers. Working with beavers instead of against them turns out to be one of the cheapest and most effective ways to manage run-off in flood-prone areas.

For the underground side of the cycle, see groundwater. For where the rain came from, see precipitation.