Desná Dam, Historical dam ruins in Mariánská Hora, Czech Republic.

Desná Dam is the remains of a failed reservoir structure in Mariánská Hora, Czech Republic, with visible fragments of a control tower and drainage channel surrounded by dense spruce forest. The foundation walls emerge from the undergrowth and still outline the original structure at the edge of a narrow stream valley.

Construction began in 1912 to supply water to the growing industrial region and was largely completed by 1915. In September 1916 the dam wall gave way under the pressure of accumulated water and released a deadly flood wave downstream.

The dam is known as Protržená přehrada (the Broken Dam), and the six-stop educational trail around it explains the 1916 disaster. A memorial plaque was placed in 1937 in Desná on a boulder carried down by the flood.

Access is on foot along marked hiking trails that start from several parking areas in the surrounding forest. The paths can be slippery when wet, so sturdy footwear is advisable for the approach through the woods.

The bats winter not in the ruined dam walls but in the 1.1 km connecting tunnel to the Souš reservoir, where eleven species have been recorded. Since 2005 this hibernation site has been listed as a site of European importance, and the tunnel entrance is closed to visitors.