Bielersee is a lake in western Switzerland. Together with Lake Morat and Lake Neuchâtel, it is one of the three large lakes in the Jura region of Switzerland. It lies approximately on the linguistic border between the German and French-speaking areas.
The lake is 15 km long and up to 4.1 km wide. Its surface area is 39.3 km² and its maximum depth is 74 m. The lake is located at an altitude of 429 m.
The catchment area of Lake Bielersee is approximately 8 305 km². Water in the lake stays in the lake for an average of 58 days. Main tributaries are the Aare and Zihl/Thielle rivers flowing out of Lake Neuchâtel, the Twannbach draining the surging first Jura mountain range and the Suze draining the Vallon de St. Imier. The Aare River was diverted into the lake in 1878 to prevent flooding of the nearby area known as the ‚Seeland‘ and drains the lake down to Büren via a canal built at the same time. Level of all three lakes is regulated by a dam built across the canal, in Porto.
The largest settlements on the banks of the Bielersee are the bilingual town of Biel/Bienne and its sister town Nidau, both located at the northern end. Tourist attraction is the island of St. The island is located near the town of Erlach/Cerlier. Jean-Jacques Rousseau once spent several months on the island.
One of the main businesses in the area is the watchmaking and associated microtechnical industries. World-famous watch brands, including the Swatch group and Rolex movements, have their premises and headquarters in the area. The lake level is the lowest point in the canton of Neuchâtel.
