The present paper studies long-term (1923–2005) changes in the hydrological regime of the medium (100–1,000 km2) river of the Ordovician oil shale field of north-east Estonia. The changing regime in the heavily mined catchments is contrasted with a morphologically similar reference catchment (River Keila) where there has been no mining activity. The Gumbel Method and Rodionov Regime Shifts Algorithm (STARS) were used to study high- and low-water changes of the mining area. The study shows that mine water has no significant impact on the River Purtse annual run-off; however, it has influence on the low water period and minimum flow values, increasing the amount of run-off. Mine water discharge can affect the course of the run-off during the high water period and high water season peaks. Therefore, in terms of the river maximum flows return period, there are no major differences between the mining and natural catchments.

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