FLOODS

They occur:

• When it rains an unusual amount [in any basin]

• When it rains an unusual amount in an already wet basin [a very wet year]

• When it rains a usual amount in a basin made more impermeable by anthropogenic actions [overgrazing, agriculture, paving]

• In arid/semiarid areas generally more intensely than in subhumid/humid areas [flash floods, killer floods]

• In areas that are geomorphogically flat [as opposed to hilly].