Rivers

Flooding happens when a river overtops its banks and water spreads across the surrounding land. Floods can be caused by heavy rainfall, snowmelt, urbanisation, or deforestation. They cause damage but also deposit fertile silt on floodplains.

A delta is a landform built up where a river deposits its load as it enters the sea or a lake. An estuary is a wide, tidal river mouth where fresh and salt water mix. Both form at the mouth of a river's lower course.

Rivers erode (wear away) and transport rock and sediment in the upper course, then deposit it when they slow down. Four erosion processes — hydraulic action, abrasion, attrition, and solution — work together to shape the landscape.

Rivers change their character from source to mouth — fast and narrow in the upper course, wider and slower in the middle course, and broad and winding in the lower course. Each section is shaped by erosion and deposition processes.

A river begins at its source — usually high in mountains or hills — and flows downhill, growing larger as tributaries join it, until it reaches its mouth where it empties into the sea, a lake, or another river.