The Tomasee (Romansh: Lai da Tuma or Lag da Toma) is a lake at the northern face of Piz Badus, above the village of Tschamut in Grisons, Switzerland. Its surface area is 2.5 ha (6.2 acres).It is the source of the Anterior Rhine and is deemed to be the official source of the Rhine (the source of the Posterior Rhine is above Hinterrhein, Switzerland, at 46.499°N 9.065°E / 46.499; 9.065 (Source of Hinterrhein)).