10 Things You Probably Didn’t Know About Lake Brienz

1. It is likely the cleanest lake in Switzerland, perhaps even "too clean" Thanks to modern wastewater treatment, Lake Brienz is ultra-oligotrophic (nutrient-poor). Phosphorus levels dropped from 20 micrograms per liter in the 1980s to below 1 microgram today. The water is so pure that it presents a paradox: there are sometimes too few nutrients to support the base of the food chain. Swiss Community

2. It acts as a massive "sediment conveyor belt" Lake Brienz receives roughly 300,000 tonnes of suspended sediment every year, primarily from the Aare and Lütschine rivers. This gives it one of the highest sediment accumulation rates of any large Swiss lake, creating the unique mineral suspension that scatters light into its famous turquoise color.

3. It is the deepest lake located entirely within Swiss borders Reaching a maximum depth of 260 meters, it holds the title for the deepest lake contained fully inside Switzerland. While Lake Geneva, Lago Maggiore and Lago di Lugano are deeper, they share their waters with neighboring France or Italy. Bathymetric map

4. It is home to the rare and endangered yellow-bellied toad The Lütschisand nature reserve in Bönigen protects a remnant delta wetland. These specific pools support one of the largest populations in the Canton of Bern for the yellow-bellied toad, a flagship species for local conservation. University of Bern / Conservation Biology

5. It hosts one of the world’s oldest operational funiculars The Giessbach funicular opened in July 1879. It remains a heritage-grade engineering marvel, originally using a hydraulic water-weight system and "Abt" passing technology to ferry guests from the boat landing to the historic hotel above. Giessbach.ch Official History

6. It was once part of a "Super-Lake" called the Wendelsee Around 10,000 years ago, Lake Brienz and Lake Thun were a single, massive body of water known as the Wendelsee. Over millennia, the Lütschine and Lombach river deltas deposited so much debris that they created the "Bödeli" (the plain where Interlaken sits), splitting the giant lake in two. Bernerzeitung

7. The water at the very bottom can remain still for years Because of its extreme depth and sheltered Alpine setting, Lake Brienz does not always "circulate" all the way to the bottom every winter. This means the deepest water can remain undisturbed and gradually warm for several years before a full mixing event occurs. Brienzersee: Ein Ökosystem unter der Lupe

8. There are roughly 280 tonnes of army munitions at the bottom Between 1948 and 1949, the Swiss Army disposed of surplus WWII ammunition in the lake. While it sounds alarming, the munitions are buried under sediments and under constant government monitoring to ensure they do not impact the water quality of the lake's sensitive environment. Federal Department of Defence (VBS/DDPS)

9. Its water levels are artificially regulated by historic weirs The lake is not fully "natural." Its levels are controlled by two weirs at Unterseen/Interlaken. This system is designed to prevent the lake from dropping too low in winter, ensuring it remains part of a managed water system that eventually feeds into the Rhine. National Centre for Climate Services (NCCS)

10. It has no verified modern record of ever freezing over completely Due to its massive volume and 259-meter depth, the lake retains an immense amount of thermal energy. Even in the legendary "Ice Winter" of 1963, when Lake Zurich became a walkable ice field, Lake Brienz remained open water. MeteoSwiss