Ahr () is a river in Germany, a left tributary of the Rhine. Its source is at an elevation of approximately above sea level in Blankenheim in the Eifel, in the cellar of a timber-frame house near the castle of Blankenheim. After it crosses from North Rhine-Westphalia into Rhineland-Palatinate. The Ahr flows through Ahr valley or Ahrtal, passing through the towns of Schuld, Altenahr and Bad Neuenahr-Ahrweiler. Between Remagen and Sinzig (south of Bonn), at about above sea level, it flows into the Rhine. The length is roughly , of which is within Rhineland-Palatinate. The Ahr has a gradient of 0.4 percent in its lower course, and 0.4 to 0.8 percent in its upper course. The Ahr and its tributaries are a main drainage system of the eastern Eifel. The watershed is approximately . History There were isolated settlements in the Ahr valley beginning at the latest in Roman times, evidenced by the Roman villa near Ahrweiler. Owing to their isolated location, the upper and middle parts of the course were sparsely populated. That began changing in the mid-19th century. The development of settlements and transportation routes (including three railway lines) and the expansion of the area used for agriculture in the Ahr valley led to the narrowing and constraint of the riverbed. The course of the river was graded and its banks were stabilized. Catastrophic flash flooding occurred on the Ahr in 1910 and 2021 resulting in the deaths of hundreds. Course The Ahr rises in the middle of the village of Blankenheim, in the Eifel region of the German state of North Rhine-Westphalia, in the basement of a timber-framed house in an enclosed spring at a height of . It then flows through the Schwanenweiher pond below Blankenheim Castle. The stream, which runs in a south-easterly direction, through the valley meadows of upper Ahr valley, has not cut very deeply into the heights of the Eifel. Much of the upper Ahr valley is protected and follows the B 258 federal road. Below the hamlet of Ahrdorf in the municipality of Blankenheim, the Ahr reaches its southernmost point and enters the state of Rhineland-Palatine. From Müsch via Antweiler to Fuchshofen, the river runs northwards and turns east at Schuld heading through Insul to Dümpelfeld. Here, the Ahr collects the waters of the Adenauer Bach coming from the right and flowing in a northeasterly direction. In Altenahr, the valley bends