The Ruhr (German pronunciation:[ˈʁuːɐ̯], German:Ruhrgebiet), or the Ruhr district, Ruhr region or Ruhr valley, is an urban area in North Rhine-Westphalia, Germany. With a population density of 2,800/km² and a population of eight and a half million, it is the largest urban agglomeration in Germany. It consists of several large, industrial cities bordered by the rivers Ruhr to the south, Rhine to the west, and Lippe to the north. In the southwest it borders the Bergisches Land. It is considered part of the larger Rhine-Ruhrmetropolitan region of more than 12 million people, which is among the largest in Europe.
From west to east, the region includes the cities of Duisburg, Oberhausen, Bottrop, Mülheim an der Ruhr, Essen, Gelsenkirchen, Bochum, Herne, Hagen, Dortmund, and Hamm, as well as parts of the more "rural" districts of Wesel, Recklinghausen, Unna and Ennepe-Ruhr-Kreis. The most populous cities are Dortmund (approx. 572,000), Essen (approx. 566,000) and Duisburg (approx. 486,000). The Ruhr area doesn't have an administrative center; each city in the area has its own administration, although there exists the supracommunal "Regionalverband Ruhr" institution in Essen. Historically, the western Ruhr towns, such as Duisburg and Essen, belonged to the historic region of the Rhineland, whereas the eastern part of the Ruhr, including Gelsenkirchen, Bochum, Dortmund and Hamm, were part of the region of Westphalia. Since the 19th century, these districts have grown together into a large complex with a vast industrial landscape, inhabited by some 7.3 million people (including Düsseldorf and Wuppertal).
The source of the Ruhr is near the town of Winterberg in the mountainous Sauerland region, at an elevation of approximately 2,200 feet (670 m). It flows into the lower Rhine river at an elevation of only 56 feet (17 m) in the municipal area of Duisburg. Its total length is 218km (135mi), its average discharge is 79 m³/s (cubic metres per second) at Mülheim near its mouth. Thus, its discharge is, for example, comparable to that of the Ems river in Northern Germany or the Thames river in the United Kingdom.