The Birsig is a rather small river in eastern France and northern Switzerland. Its source is in the village Biederthal, in the French Haut-Rhin department, near the Swiss border. The Birsig is about 21 kilometres long, and its watershed area is about 82 square kilometres. It flows variably through Swiss and French territory and through the Birsig Valley. Afterwards it passes the city of Basel, where it enters the Rhine (left bank).
