These impressive sluices on the Amstel river, near Koninklijk Theater Carré, date from 1674 and are still in use to today. They allow the canals to be flushed with fresh water from lakes north of the city, rather than salt water from the IJ River, an innovation that made the city more livable. The locks are shut while fresh water flows in, while the sluices on the western side of the city are left open as the stagnant water is pumped out to sea.