Ismaning Reservoir

A wastewater lake changes its feathers?

At the Ismaning Reservoir approximately an hour by bike northeast of Marienplatz the interplay between humans and nature is evident. It is not possible to swim in the lake. But it does more than just store water for Munich power generation facilities. It also provides habitat for many species.

The western part of the lake (Westbecken) during the winter.

There is no boat rental, no beach, no kiosks or tourism. Built in 1929, the reservoir and the canal connecting it to the Isar River help to regulate the water flow to the nearby hydroelectric plants and for cooling the turbines of the thermal power station Heizkraftwerk Nord. The 95 adjoining ponds of the fish farm Birkenhof purify the city’s wastewater through natural means.

Left glass: Large-scale carp breeding started in the wastewater lakes in the 1920s. The wastewater contained so many nutrients that the animals did not need any additional feed.
Right glass: With advancements in technology, Munich’s wastewater became cleaner in the 1990s. As a consequence, however, the water contained fewer nutrients.

What fascinates me about the Reservoir is the multitude of different aspects that are coming together: For instance the progressive energy project, the wastewater treatment, the decades-long pond farming, the large bird reservoir, and the positive influence on the groundwater and the climate.
—Rudolf Lukes, local resident

Technological progress has altered the ecosystem of the lake and the ponds and unintentionally created the largest moulting grounds for water birds in central Europe. But is it possible for the wastewater in the ponds to be too clean? And why is the thermal power station good for the rare mercury bluet?

Although the reservoir is almost the same size as the Tegernsee, which is a popular tourist destination, most people from Munich have never heard about the Ismaning Reservoir.

The dream of electricity from the Isar

The construction of the reservoir was part of a grand vision: the electrification of all of Bavaria. Prior to the construction of the first hydroelectric plants in 1896, only mill wheels were used to produce energy. For the first time, the Ismaning Reservoir and its respective power plants were supposed to supply energy to half a million households in Munich.

A grand vision: the electrification of all of Bavaria

Oskar von Miller, the founder of the Deutsches Museum and a visionary of hydroelectric power, initiated the construction of the Ismaning Reservoir.

This image is in the public domain. It can be found here.

The setup of the Ismaning Reservoir station during the Ecopolis München 2019 exhibition.

Photograph by Laura Kuen. CC BY 4.0 international license.

A demonstration of hydroelectric energy powering a light bulb during the Ecopolis exhibition.

A demonstration of hydroelectric energy powering a light bulb during the Ecopolis München 2019 exhibition.

Photograph by Laura Kuen. CC BY 4.0 international license.

Wildlife

The Unterföhrung thermal power station cools its turbines with the water from the Isar canal. Afterward, the heated water reaches the Ismaning Reservoir. This contributes to the fact that the lake does not freeze over—not even during the deepest winter. One species particularly appreciates this: The mercury bluet (Coenagrion mercuriale), which is a protected dragonfly species, finds ideal living conditions because of the existence of the thermal power station.

A mercury bluet (Coenagrion mercuriale).

In order to create the reservoir a large area of peat bog was drained. Today such a major alteration of the landscape would be met with immense resistance. In addition to the marshland, many villages were destroyed by the project of creating the artificial lake.

My father was a fisherman at the fish ponds. The working conditions were extremely difficult, particularly during the wintertime when the fishing began before Christmas.
—Gerda Lukes, local resident

Many people lost their land as a result of the construction of the reservoir.

For others, the power station and the fish farming meant hard work, but also new livelihoods.

Where does the water in the reservoir come from? In Oberföhring 150 m³ of water per second is diverted from the Isar through the Mittlerer Isarkanal. As a result of the reduced water level in the Isar, many fish species that once lived in the river have now vanished.

The dream of energy drove the fish out of the Isar; however, the creation of the reservoir also inadvertently created a new habitat for countless birds. In July 2015, a counting showed that 60,000 birds were present at the Ismaning Reservoir.