Flood Generation Mechanisms and Changes in Principal Drivers

Mechanisms generating floods are reviewed and next discussed with regard to the Upper Vistula Basin. Here, floods typically result from (i) moderate-intensity rain that lasts a few days over a large area and drives large-scale flooding, or (ii) high-intensity, short-lasting convective rain causing local flash floods. Outside the mountain part of the basin, especially in the San River catchment, floods are also caused by intensive snowmelt. Interpretation of climate track in flood generation is presented, based on the analysis of observation records from the last six decades and projections for the future. Catchment and river changes affecting the conditions of flood generation are next considered for the last 130 years. They comprise changes regulating flood runoff (catchment reforestation and dam reservoirs construction), changes reducing floodwater storage and accelerating flood runoff (channel regulation, flood embankments, river incision, and permanent impoundment of the Upper Vistula for navigation purposes), as well as the expansion of riparian forests increasing large wood recruitment to channels during floods.