What causes floods?

The interaction of the shape of the area, saturation of the soil, vegetation, available bedload, rate of snow coverage as well as technical intervention in water courses (e.g. dam, retention basin and so on) has a determining influence on the likeliness of flood in a catchment area. Extreme rainfall events, which are either long lasting or of high intensity can trigger floods. A strong snowmelt also can contribute to flood. 

Floods act differently, depending on the channel system. In torrent systems intense bed load transport and deposition dominate. Here, one often finds debris flow, bank erosion and overbank sedimentation. In watercourses in the valley bottom, however, flooding become rife, but bank and bed erosion are also possible which leads to undercutting. Besides, bed aggradations might occur aswell.