The source of the river is in the southern Teutoburg Forest in North Rhine-Westphalia. In Lower Saxony, the brook becomes a comparatively large river. Here the swampy region of Emsland is named after the river. In Meppen the Ems is joined by its largest tributary, the Hase River. It then flows northwards, close to the Dutch border, into East Frisia. Near Emden, it flows into the Dollart bay (a national park) and then continues as a tidal river towards the Dutch city of Delfzijl.