Its source is located in the Austrian state of Vorarlberg, where the river rises from lake Formarinsee in the Alps at an altitude of . It flows in a north-north-easterly direction and crosses the German border, forming the Lechfall, a waterfall; afterwards the river enters a narrow gorge (the Lechschlucht). Leaving the Alps, it enters the plains of the Allgäu at Füssen at an elevation of in the German state of Bavaria, where it used to be the location of the boundary with Swabia. The river runs through the city of Füssen and through the Forggensee, a man-made lake which is drained in winter. Here, it forms rapids and a waterfall.