100 years of river flow timing characteristics and extreme flow analysis

River ice has a significant impact on nearly 60% of rivers in the northern hemisphere where ice jams due to river ice are responsible for some severe and recurrent floods in the northern rivers. The ice formation (i.e., freeze-up event) occurs during the wintertime when the temperature drops to freezing point and when the flow gradually reaches the lowest value for a while, and it keeps the river frozen with low flow occurring close to the period of the maximum ice cover before the spring melt is initiated.  In this research, we focus on detecting the periods with low flow (i.e., freezing period). Daily discharge time series were used to derive the annual freezing periods as well as extreme discharge values over a century in Tana and Tornio Rivers in the Finnish borders of Sweden and Norway.  Therefore, the timing characteristics such as duration, probable shifts through time, and overall flow extremes including the average low and high flow in a period of 90 days in each water year were quantified. The study showed that both low and high flows in two rivers had a significant negative trend in their occurrence date by a confidence level of 95%. In addition, it was observed that the seasonal 90-day low and high flow periods happened earlier in recent years. On the other hand, Tana River showed a negative trend in its annual minimum flow over the century which is an opposite event in comparison with Tornio River. The duration of low flow in the Tana River has been significantly increased by the confidence level of 95% from a range of 50-70 days to a range of 100-140 days. In Tornio River, the duration has been significantly decreased by the confidence level of 95%. In the first ten years, the duration is about 120 days on average, while the duration in the last ten years is about 50 days.