Möhne dam: Ruhrverband starts deep water aeration

On Monday, 8 September 2025, the Ruhrverband will begin laying oxygen pipes in the main basin of the Möhne Dam. The precautionary aeration of the deep water will begin on Tuesday to prevent possible fish mortality.

From the start of the measure, fishing will be prohibited in the area of the oxygen enrichment plant. The imaginary line between two bays marks the boundary:

- north bank: first bay as seen from the dam wall, level with the Möhnesee yacht club
- south bank: second bay as seen from the dam wall

Towards the dam wall, this line may only be crossed without angling activity. It is also strictly forbidden to moor boats to the orange-yellow buoys that mark the area.

Precautionary measure for around four weeks
The aeration of the deep water is purely a precautionary measure that the Ruhrverband has already successfully implemented at several dams in recent years. Based on previous experience, aeration will take around four weeks. The fishing ban in the areas mentioned will also remain in place for this period.

Background: Why there is a lack of oxygen
Stable temperature layers form in the reservoir between spring and autumn:

- Cold, heavy water at the bottom
- Warm surface water due to summer warming
- A so-called thermocline in between

In the thermocline layer, decomposition processes of dead phytoplankton and zooplankton consume a lot of oxygen. This creates an almost oxygen-free "bar" that traps the cold-loving fish species at the bottom of the lake. If the oxygen content there falls below three milligrams per litre, the fish stocks are in danger. In order to prevent this, the reservoir operator, the co-operation laboratory and the fisheries management of the Ruhrverband have been monitoring the values closely since the end of the summer. Targeted aeration has successfully prevented fish mortality in recent years.

The stable temperature stratification usually dissipates again from October due to stronger autumn winds, cooler air temperatures and increased inflows to the dam.

Water quality remains high
The low oxygen levels measured are a natural phenomenon and are not related to the water quality. This remains at a high level in the Möhne Reservoir - as in all other Ruhrverband reservoirs.