Sélestat lies on the Alsace plains, a narrow, very fertile area that stretches between the Rhine and the Vosges mountains. The Ill flows parallel to the Rhine and crosses Sélestat. This river frequently branches and swells, making the area very wet and subject to flooding. Sélestat is only from the Vosges, at the opening of one of the rare valleys crossing the mountain range and providing a connection to the rest of France. This valley corresponds to the course of the Giessen, a long tributary of the Ill. In contrast to the Ill, which has a constant annual flow, the Giessen is a mountain river subject to sudden increases in the water level, especially during the spring thaw period. The Giessen passes north of the town and meets the Ill several kilometers to the east, in Ebersmunster.