Another meteorological risk, although one that occurs less often, is that related to the rapid melting of snow lying in the catchment. This can be a result of a sudden rise in temperature after a prolonged cold period, or when combined with extensive rainfall. Many of the largest historical floods were caused by snowmelt, but the last such episode occurred when the bitter winter of 1946-7 was followed by a rapid thaw due to rain in March 1947 and caused severe flooding all along the Trent valley.