Lake Biel

Between the Jura slopes and the Seeland, the shoreline also tells a story of hydraulic engineering. The first Jura water correction lowered the lake and linked St Peter's Island to the mainland along the Heidenweg. The resulting shallows and wetlands provide valuable habitats. A historical shoreline therefore cannot simply be equated with today's map or a current level reading.

The Aare enters through the Hagneck Canal, with further water arriving from the Schüss and the Zihl Canal connecting Lake Neuchâtel. Water returns to the Aare through the Nidau-Büren Canal. Since the river diversion, the catchment also includes Alpine regions upstream. The substantial throughflow makes the transport of water, suspended material and nutrients particularly important to this lake.

The Port regulating weir controls the outlet of the three Jura lakes according to seasonal objectives. Rainfall and snowmelt in the Aare catchment therefore interact with regulation to shape the level. Conditions downstream matter too: during an Emme flood, discharge at Port may be reduced even while the lakes rise. Regulation manages limited storage capacity; it does not prevent every flood.

The lake level, the incoming Aare at Hagneck and the outlet discharge at Port are three different quantities. A rising lake alongside high outlet discharge is not a contradiction: even more water may be entering than leaving. Compare timestamps and trends across the available stations. A single inflow reading describes neither the complete water balance nor, by itself, a hazard level along the shore.

Rapid water renewal does not mean Lake Biel is mixed all the way to the bottom throughout the year. Warm upper layers can develop above cooler deep water in summer; wind and cooling later encourage mixing. FOEN's 2016 factsheet describes temporary deep-water oxygen deficits despite reduced phosphorus levels. A pleasant surface temperature therefore tells us little about living conditions near the lakebed.