The causes of flooding can be broken down into: climatic, physical and human.
Heavy rainfall over a short or long period of time
Ground that is saturated as a result of previous rainfall
Melting snow and glaciers
Soil baked hard by the sun therefore acting as if impermeable
Hurricanes causing wave surges
Narrow steep-sided valleys causing surface run-off to reach rivers rapidly after a storm
Impermeable rock causing rapid suface run-off
Low-lying coastal areas
Tsunamis
Urbanisation can lead to an increase in tarmac and drains, which causes rapid surface run-off
The diversion of a river or the narrowing of a channel can lead too flooding
Deforestation in the river basin leads to less water being taken up by roots
Deforestation and the loss of tree roots mean that the soil is loosened and when rain falls and travels as surface run-off, it takes with it soil that washes into the river and silts up the river, displacing water