Altarmwehr Quitzöbel

The Gnevsdorf receiving water is an approximately 11 km long canal between the Havel and the Elbe. It is an artificial Havel drain and is used to move the backwater point of the Havel downstream. This flood protection system includes the four weirs at Quitzöbel (old arm and piercing weir), Neuwerben and Gnevsdorf as well as the Havelberg lock.
Source: Wikipedia

there is a whole series of locks here

In total, the Quitzöbel defense group consists of four defense systems. It is the last barrage of the Havel. The Quitzöbel defense group protects the Havel lowlands from Elbe floods, serves to cut off a flood crest of the Elbe, regulates the water level of the Lower Havel and enables the Elbe to increase in low water levels.