This paper discusses observed changes in the hydrological regime in Finland. The aspect of article is to look at the effect of chosen time period on trend slopes. It is recognized that trend analysis is highly dependent on the chosen time period. Especially starting and ending years have a huge effect on the trend slope. Both non-parametric Mann-Kendall trend test and parametric test are used in this paper and compared with each others. The lake surface water temperature, lake ice cover and discharge time series are studied. All these variables have records at least since the early 20th century. Some ice break-up and discharge time series are available from the mid-1800s. Long-term time series show winters becoming milder, thus the winter discharges have increased. Ice freeze-ups have become later, break-ups earlier and ice cover duration shorter. Lake surface temperatures have risen lately, although no extensive warming trends were found until the end of the 20th century. For which time period the trends are strongest? The most recent couple of winters have been very warm in Finland, how is this affecting trend slopes?