tokens: The peak is bounded by the Bistrishko Branishte Biosphere Reserve to the east , and a prohibited drinking water catchment area to the west , and is part of the divide between the Black Sea and the Mediterranean , with the Vitoshka Bistritsa draining its eastern slopes to flow into the Iskar River and eventually to the Danube River , and the Struma River draining the western slopes to flow into the Aegean Sea .