River Flow Analysis—It Is So Easy, but It Isn’t?

Scientists, engineers, students, as well as people who are not dealing with hydrology very often use the term “water level is rising” or “flow is increasing.” This encompasses few logical steps in defining a trend, which is in most of the cases linear. Such procedures give “first hand” predictions of what will happen in the future. It should be emphasized that the presented procedure could lead to the wrong conclusions. This topic will be addressed in the paper as an example of the average daily flow time series for the available duration of 20 years, from the year 1999 till the year 2018. Analysis will be presented on a real case study of two hydrological measuring stations on River Bednja in Croatia. These are Lepoglava (most upstream) and Ludbreg (most downstream measuring stations).