Lungernsee. Lake Lungern is a natural lake in Obwalden, Switzerland which is named after the town Lungern on its shore. The lake is drained by the Sarner Aa river, which flows through the Sarnersee and into Lake Lucerne. The lake was originally much larger, and covered a large part of the valley it is situated in. Starting in 1836, its level was lowered by 36 metres through an artificial drainage tunnel with a length of 380 metres. It is used as a reservoir.