Until the early 1980s, industry was a major source of water pollution. Although many plants and factories can be found along the Rhine up into Switzerland, it is along the Lower Rhine that the bulk of them are concentrated, as the river passes the major cities of Cologne, Düsseldorf and Duisburg. Duisburg is the home of Europe's largest inland port and functions as a hub to the sea ports of Rotterdam, Antwerp and Amsterdam. The Ruhr, which joins the Rhine in Duisburg, is nowadays a clean river, thanks to a combination of stricter environmental controls, a transition from heavy industry to light industry and cleanup measures, such as the reforestation of Slag and brownfields. The Ruhr currently provides the region with drinking water. It contributes 70 m3/s (2,500 cu ft/s) to the Rhine. Other rivers in the Ruhr Area, above all, the Emscher, still carry a considerable degree of pollution.