A series of locks regulates the flow of the Amstel River through the canals and into the North Sea. By the way, in 1396, the Dutch invented the first true modern lock. Called the pound lock, it uses two gates on either end of a chamber (the “pound”). Boats wait in the pound for the gates to raise or lower the water level so it matches that of the river or bay on the other side. It’s a system we’re intimately familiar with in Chicago, as this is how water levels get regulated for traffic coming down the Chicago River and entering Lake Michigan.