Tomasee is a lake

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

 at the northern face of Piz Badus

Piz Badus

The Piz Badus or Six Madun is a mountain in the Lepontine Alps, lying on the border of the cantons of Uri and Graubünden.In its northwestern face, which is the Graubünden side, lies Tomasee, source of the Anterior Rhine....

, above the village of Tschamut in Grisons, Switzerland

Switzerland

Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

. Its surface area is 2.5 ha

1 E4 m²

To help compare orders of magnitude of different areas, areas between 1 hectare and 10 hectares are listed below.* Areas smaller than 1 hectare* 1 hectare is equal to:** 1 E+4 m² or 10,000 m²** 100 ares** 108,000 square feet...

.

It is the source

Source (river or stream)

The source or headwaters of a river or stream is the place from which the water in the river or stream originates.-Definition:There is no universally agreed upon definition for determining a stream's 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

Hinterrhein, Switzerland

-External links:...

, at 46.499°N 9.065°E).

It is possible to reach the lake on a path from Oberalp Pass

Oberalp Pass

Oberalp Pass is a high mountain pass in the Swiss Alps connecting the cantons of Graubünden and Uri between Disentis and Andermatt.-Winter closure:...

, suitable for most walkers although still a mountain trail.