The Nature reserve „Przełom Sanu nad Grodziskiem” (the San River Gorge by Grodzisko) established in 2003 on area of 100.24 hectares within three villages: Zwierzyń (Olszanica commune), Bereźnica Niżna (Solina commune) and Średnia Wieś (Lesko commune) in Lesko district. It covers an attractive landscape of water gap section of the San River Valley and the area by the Myczków Reservoir below the power plant in Zwierzyń. The area in this place is overgrown with mixed forest with subcontinent oak-hornbeam forest, fertile Carpathian beech forest and mountain sycamore, in which there is a remarkably valuable position of hart’s-tongue.
Flora of plants is very plentiful here, and among them are sporadic and protected plants such as broadleaf and violet helleborine, monkshood Moldovanor, Europenscopolia. Over 130 species of birds have been found here, most of which are breeding.
The green trail between Zwierzyń and Myczków runs along the eastern border of the reserve.