Oberaargau and the Unteraargau and ruled them as one. When Bern lost its old power in 1798, the Unteraargau became the modern canton of Aargau. The boundary between the Oberaargau and Unteraargau was fixed in 1798 along the Wigger River. In 1802, however, it was shifted west to Murg river. This action was confirmed by the Act of Mediation of Napoleon in 1803 and the Congress of Vienna in 1815. References Notes Citations External links Regional Web site (in German) Regions of Switzerland Geography of Switzerland