Schutsluis and weir lock, so-called "Oude Sas" on the Dender.
In 1667-1669 a lock with four new state water mills was erected on the Dender in the center of the city, to replace the locks and the destroyed mills of the Ooie-Nieuwburcht. The sluice with sludge played a role in the military defense of the city to flood the surrounding meadows. The new construction of the lock consisted of a central navigation channel between two brick lock walls covered with gray sandstone and equipped with pointed icebreakers, founded on a wooden floor and wooden foundation piles. The lock for inland vessels was closed with six wooden lock gates. On the lock walls stood five brick mill buildings, connected by a wooden footbridge, over the lock was a movable wooden footbridge. The mills were demolished in 1852-1853. The lock, modified in 1768 after the canalisation of the Dender between Aalst and Dendermonde, remained in use as a weir until November 15, 1978, when the Dender estuary was relocated. The old lock construction is now being restored. A new bicycle and pedestrian bridge, an iron swing bridge next to the lock is in use.
Lock with brick lock heads and gully walls, founded on piles and covered with natural stone. Two parallel elongated lock heads with pointed tip. Double wooden lock gates that were opened and closed via wooden capstans. The weir lock is operated by means of racks (toothed rods) driven by vertical winches.
During the restoration work on the lock, an old floor of oak beams was discovered at the bottom of the turning basin behind the weir lock, probably dating from the 17th century.