The Salek valley used to be exposed to huge amounts of pollutants due to its close vicinity to the largest Slovene thermal power plant of Sostanj (STPP). Due to large emissions of SO2 and heavy metals as well as dumping of fly ash negative effects on the environment appeared (e.g. forest decline in area exposed to deposition of emission by the STPP, pollution of lake Velenje and the river Paka). Therefore, several ecological remediation measures on the STPP were implemented in the 1990s, and several research projects on reasons and effects of forest decline and degradation of environment began as well. A continuous and marked improving of the condition of both forest and freshwater ecosystems (of lake Velenje and the river Paka) after the installation of desulphurization devices on Units 4 and 5 of the STPP and construction of a closed loop system for the ash transportation is emphasized in the present paper. nema