A flood is usually unusually heavy flow in a stream, great enough that it exceeds the capacity of the streambed. Floods are associated with heavy rain and spring runoff. Another possible cause of a flood is the breaking of an artificial or natural dam, perhaps one caused by a landslide.
The rise in the level of any body of water may flood surrounding land, thus as a reservoir is filled the land within it is flooded. Likewise a storm surge during a hurricane may flood the shore of a sea.
The incoming tide is called the flood tide.