Belmeken-Sestrimo-Chaira Cascade, Hydroelectric cascade in Eastern Rila Mountains, Bulgaria

The Belmeken-Sestrimo-Chaira Cascade is a hydroelectric system in Eastern Rila Mountains consisting of three reservoirs and two pumped-storage power stations that manage water from the Maritsa, Struma, and Mesta river basins. The cascade spans different elevations across the mountains, with the main reservoir situated at 1923 meters.

Construction of this engineering complex began in 1964 and continued through multiple phases until 1999, establishing it as Bulgaria's largest hydrotechnical facility. The gradual development allowed for expanding hydropower generation capacity across the region over several decades.

The cascade's name links three sites: the Belmeken reservoir and pumped-storage plant, the Sestrimo plant, and the Chaira pumped-storage plant. The Chaira machine and transformer halls sit 350 metres underground in purpose-built caverns, reached by two 1.7-kilometre pressure pipelines from the Belmeken reservoir. The reversible tunnel Beli Iskar, 2.8 km long, connects the Grunchar diversion with the Iskar cascade and carried water to Sofia between 1983 and 2000.

Visitors can access the cascade through marked trails from Sestrimo village or the Yundola area. Weather conditions and seasonal changes affect accessibility and the best viewpoints for seeing the reservoirs.

The Chaira pumped-storage power plant is one of the largest of its kind in Southeast Europe and uses elevation differences between the reservoirs for power generation. This innovative system pumps water uphill during surplus energy periods and releases it later to produce electricity.