In the 60-s two dams with storage reservoirs were constructed on the San River: the largest Polish reservoir in Solina, and the one in Myczkowce, where a complex of hydro power plants was built. Designing hydrotechnic structures is very difficult as their environmental impact has to be taken into account - any reservoir or dam must be an integral part of the environment, and environmental and economic needs are equally important. The author discusses the situation in the Bieszczady Mts. after completion of the dams.