The barrier lake Witznau in the community of Ühlingen-Birkendorf in the state of Baden-Württemberg. The barrier lake and valley lock is a pump accumulator lake in the Schwarza valley in the Black Forest. It is the lower pool of the pumped storage hydro power station of the Schluchseewerk AG between the lake Schluchsee and the river Rhine. The upper pool is formed by the barrier lake Schwarzabruck