Speaking of the Grimsel Pass:
The Grimsel Pass is in the heart of the Alps, an exceptional natural area. From a hydrological point of view, the large amounts of precipitation are unique. Here at the Grimsel Pass, this is an average of around 2300 litres per square metre per year.
300 litres of this evaporate per square metre, and 2000 litres per square metre leave the region as runoff in the Rhone or Aare. That is a lot of water: twice as much as the Swiss average and almost seven times as much as the average in Germany.
Depending on which side of the pass the water drops reach the earth's surface, they flow into the Mediterranean or the North Sea.
The Aare, with a total length of around 290 km, flows through the cantons of Bern, Solothurn and Aargau, crossing Lake Brienz, Lake Thun and Lake Biel. It is the longest river located entirely in Switzerland. Although the Aare carries more water on average at the confluence with the Rhine, it loses its name. After another 1000 km of flow, the Aare water flows into the North Sea as part of the Rhine. The Rhone, on the other hand, keeps its name until it flows into the Mediterranean. Its approximately 810 km long flow takes it through the Valais into Lake Geneva and from there via Lyon into the French Rhone Valley, where it repeatedly receives tributaries from the Western Alps. While the Aare flows into an oceanic climate, the Rhone carries the water from the Grimsel Pass into the Mediterranean climate zone, where the abundant snow and glacier meltwater from alpine rivers is very popular in the dry summers.