Rhine hits 28°C, Mosel oxygen plummets as German rivers overheat, threatening fish populations

Water temperatures surge across German rivers

An intense heatwave has driven water temperatures in major German rivers to levels dangerous for aquatic life. The Rhine reached 28 degrees Celsius at Koblenz, where the already warm Mosel flows into it, according to measurements by the Federal Institute of Hydrology's Undine platform at 7:00 a.m. on Friday. The Mosel itself registered 27.8°C at Palzem and 27.6°C at Fankel. Farther downstream, the Rhine measured 27.6°C at Mainz and 27.0°C at Worms.

Oxygen levels drop below critical mark

The high temperatures have slashed dissolved oxygen concentrations, particularly in the upper Mosel. At Palzem in the Trier-Saarburg district, oxygen fell to 2.6 milligrams per liter, below the 3 mg/L threshold considered critical for fish, the environment ministry in Mainz said. The Main river is also affected; early-morning measurements at Frankfurt's Osthafen showed 27°C, surpassing the initial warning threshold of 25°C where the river's capacity to absorb oxygen declines sharply.

Authorities warn of fish die-off risk

With oxygen levels so low, stress on fish and other organisms is acute, and additional oxygen-depleting processes could tip the balance. "An der Obermosel können wir ein Fischsterben nicht ausschließen," said Rhineland-Palatinate agriculture and environment minister Christine Schneider (CDU). She explained that persistently high air temperatures, long daylight hours and intense UV radiation all contribute to rising water temperatures and falling oxygen levels, disrupting the ecological balance.

Governments urge immediate action

The Darmstadt regional council called on residents and industries along the Main to minimise heat inputs and residual pollution from wastewater discharges, and to maximize oxygen enrichment in the waterway. Similar warnings apply across the state, as the heatwave shows no sign of ending soon.