The groundwater-induced flooding that occurred in the Somme Basin during April 2001 damaged numerous dwellings and communication routes, and economic activity of the region was flood-bound for more than 2 months. It was the first time that such a sudden event was recognized as resulting from groundwater discharge, despite the Somme valley not being prone to flooding. Because of a dual porosity of the chalk in the basin, nonlinear processes, involving a hydraulic continuity between the macropores of the unsaturated zone and the chalk groundwater, govern water migration through the unsaturated zone. Such a process is the result of switching behavior of groundwater recharge from matrix flow to macropore flow due to accumulated wetness over several years. There is much evidence to support that the flood probability model is climate-dependent for the studied region because nonlinear processes amplify the effects of nonstationarities of climatic inputs. An estimation of the return period of catastrophic flooding depends on the long-term precipitation fluctuations. This has implications for flood risk assessment requiring the need to distinguish between short- and long-term flooding risks. Other basins that may not appear particularly prone to flooding could also be subjected to similar groundwater-induced flooding should the long-term precipitation fluctuations observed in the north of France since the beginning of the 1980s persist. Similar extraordinary situations can occur in Belgium and England, whereby significant flooding results in substantial contribution of groundwater flows.