The Vorderrhein (English: Anterior Rhine; in the local Sursilvan language: Rein Anteriur) is one of the two sources of the Rhine. Its catchment area of is located predominantly in the canton of Graubünden (Switzerland). The Vorderrhein is about long, thus more than 5% longer than the Hinterrhein/Rein Posteriur (each measured to the farthest source). The Vorderrhein, however, has an average water flow of ,, which is less than the flow of the Hinterrhein. According to the Atlas of Switzerland of the Swiss Federal Office of Topography, the source of the Vorderrhein– and the Rhine –is located north of the Rein da Tuma and Lake Toma.