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The purpose of the Słup reservoir, the largest in the Land of Extinct Volcanoes, is to ensure adequate flood protection. It was built in 1974–78, when an earth-concrete dam was built across the Nysa Szalona river, just upstream of the village of Słup. It is 940 m long and 21 m high. The resultant lake is nearly 5 km long when full, occupies 4.9 km2 and is up to 20 m deep. The reservoir is also used as an angling place, which is favoured by its first class of water purity. It is a bird sanctuary, especially its shallow eastern part, with wetlands and reed communities. During seasons of bird migrations as many as 80 different species of water birds are recorded.
The construction of the reservoir caused flooding of two villages in the eastern part of the contemporary lake. In a small village of Brachów there was a palace, where the Prussian field marshal Blücher planned military activities before the Battle of Kaczawa in August 1813. Brachów joined the village of Żarek. Some buildings of this one were located above the level of the lake, nevertheless they were also demolished. Today one can see brick and stone foundations of some of them and a high cross at the road fork in the upper part of the vanished village recalls its existence. Both villages are commemorated in the names of lake shores. The northern one is called the Brachów Shore, the southern one is the Żarek Shore.