River Flooding Explained

How River Flooding Works

River (fluvial) flooding occurs when a river's water level exceeds its banks due to prolonged rainfall, snowmelt, or a combination of factors.

Causes

• Prolonged rainfall: Days of steady rain saturating catchments
• Intense storms: Flash flooding in smaller, steeper rivers
• Snowmelt: Rapid thawing adding large volumes of water
• Blocked channels: Debris, ice, or vegetation restricting flow
• Tidal locking: High tides preventing river water from draining to the sea

The River Catchment

River flooding depends on the entire catchment — all the land that drains into a river. Rainfall miles upstream can cause flooding downstream hours or days later.

Warning Time

Larger, slower rivers (Thames, Severn, Trent) can provide days of warning. Smaller rivers and streams may only give hours of notice.

The EA Warning System

• Flood Alert (yellow): Flooding is possible. Be prepared.
• Flood Warning (red): Flooding is expected. Immediate action required.
• Severe Flood Warning (dark red): Severe flooding. Danger to life.

Monitoring Stations

The EA operates thousands of river level monitoring stations across England providing real-time data used to forecast flooding and issue warnings.