| Summary: | We analyzed observations on floods in rivers located in Finland and northern Russia where hazardous floods often happen during a spring flooding period. We evaluated the length of spring flooding periods, the volume of spring floods, the yearly maximum water discharges (annual floods) and their dates from hydrographs. The hydrographs were evaluated using the daily water discharges given in yearly books published by the national hydrological services. The long term time series of annual and spring floods were used to define shifts (step changes) by applying the moving window technique. Three statistical criteria namely the Student test, the Kolmogorov-Smirnov test and the Mann-Whitney test were used. Our results suggest that the annual floods were recorded in the spring flooding period in more than 85 % of the rivers selected. In the last two decades, the number of annual floods that happened in autumn-winter season increased almost twice in the southern Finnish rivers. The melting snow remains the dominant source for the highest floods in the rivers located in northern Finland and Russia. The step changes were defined in half of the time series of the annual floods and spring floods. In over a one-third of the records of the spring floods, the step changes dated to the late 1990s, since then the volume of floods increased by 21 % on average. The step changes in the records of the annual floods dated to the early 1950s, mid 1970s and early 1990s.
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