Together with the Danube it formed most of the northern frontier of the Roman Empire and since those days has been a vital navigable waterway, carrying trade and goods deep inland. The Rhine's origins are in the Swiss Alps in the canton of Graubünden, where its two main initial tributaries are called Vorderrhein and Hinterrhein. When leaving Graubünden, the Rhine flows north to form the frontier with Liechtenstein and then Austria, and then empties into Lake Constance. The Rhine then re-emerges, flows west, mainly on the border between Switzerland and Germany, falls over the Rhine Falls, and then turns north at the so-called Rhine knee in Basel and forms the southern part of the border between Germany and France in a wide valley, before entering Germany exclusively.
The Rhine then turns west into the Netherlands, where together with the Meuse it forms an extensive delta. Crossing the border into the Netherlands, the Rhine is at its widest, but the river then splits into three main distributaries: the IJssel, the Waal and the Nederrijn (Nether Rhine). From here the situation becomes more complicated, as the name "Rhine" no longer coincides with the main flow of water. Most of the Rhine water flows further west through the Waal and then via the Nieuwe Waterweg and, merging with the Meuse, through the Hollands Diep and Haringvliet estuaries into the North Sea. The IJssel branch carries its portion of the water north into the IJsselmeer while the Nederrijn flows west parallel to the Waal.
At over 1000 km, the Rhine is the longest river primarily within Germany. It is here that the Rhine encounters some of its main tributaries, such as the Neckar, the Main and the Moselle. Though many industries can be found along the Rhine up into Switzerland, it is in the Ruhr area that the bulk of them are concentrated, with all of its streams — chief among them the Ruhr itself — draining into the Rhine, causing decreasing though still considerable pollution.