Effect of spring floods on water acidity in the Kiiminkijoki area, Finland

Acidity of the weakly-buffered river Kiiminkijoki and its tributaries increases considerably in spring. In Nuorittajoki, the largest tributary, it may fall to pH 5.0, while it is pH 6.4-6.8 just before the flood and over pH 7.0 (maximum 7.8) in summer. Similar spring acidification of running water occurred at all other sites studied. Increased acidity was due to abundant melt water, generally at pH 4.5-5.0. River pH varied greatly at other times of the year. A strong positive correlation between pH and water level of the river was partly due to the acidifying effect of rain and melt water, and to the abundance of peat in the area.