The river Isar, located in the south of Germany, is an Alpine river. It flows from its source in Austria at 1600 m above sea level through the Alps, pre-alpine moorlands and through the city of Munich before entering the Danube river near Deggendorf at 300 m above sea level. With a length of 260 km it is the fourth largest river in Bavaria and the second most important tributary of the Danube in Germany.

The catchment area of the Isar covers 8960 km² in the Alps and the Karwendel mountains. The Isar is generally divided into three sections: the Upper Isar, the Middle Isar and the Lower Isar. The Test Case hydropower plant Altheim is located in the Lower Isar, at km 67.2, near the town Altheim. Downstream of the Test Case, 7 additional hydropower plants are located along the Isar.
The section of the Isar that relates to the Test Case Altheim has a catchment area of 88.2 km2 and is about 73 km long. It has an intern-annual discharge of 163 m³/s. It is classified as a waterbody with poor ecological status.

Altheim hydropower plant

The hydropower plant at Altheim, commissioned in 1951, is a block-type hydropower plant and part of a chain of power plants in the Isar. It has an installed capacity of 17.8 MW and a mean annual output of 91.4 GWh. The plant has 4 weirs located on the right of the power house and a head storage volume of 1,980,000 m³.

The normal operating level is at 384.00 m above sea level, in swelling operation it can be lowered by 1.00 m. On average, there are 2 peaks per day, where the flow changes roughly between 50 m³/s and 170 m³/s with ramps of about 100 m³/s per hour or a change of water level between +45 cm/h to - 30 cm/h.

The area around the power plant is dominated by commercial and living areas on the left side and a popular local recreation area with lakes for swimming on the right side, as well as a sewage treatment plant in Altheim.

Layout of HPP Altheim

Layout and surrounding area of the HPP Altheim (google maps)

Country Germany
River Isar
Operator UNIPER
Capacity 17.8 MW
Head 8.2 m
Inter-annual discharge 163 m³/s
5 Kaplan turbines

The Operator: Uniper SE
Uniper Hydro Germany operates 109 hydroelectric power plants, containing run of river plants, storage and pumped storage. Alongside the rivers Lech, Isar, Danube and Main more than 1300 kilometres are used for the production of clean electricity. In close cooperation with regional stakeholders (politicians, authorities, NGOs) Uniper takes over responsibility to protect civil areas from floods, but also to protect the environment. Read more

Challenges at the Test Case

Pressures on the water body's ecosystem
The main pressures on the water body are nutrients, river specific pollutants, agricultural production and hydromorphological changes. These are on the one hand related to agriculture on the other hand the hydromorphology is impacted by the sequence of hydropower plants and their related ponds. Moreover, the chemical pressure (especially mercury) cannot be neglected.

Fish population
The relevant water body is located in the barbel region (epipotamal) and is therefore dominated by potamodromous species. During electro fishing in autumn 2007, 43 fish species could be identified, containing 35 indigenious species, 27 red listed species and 9 FFH species, which were mainly rheophile fish. Out of those, six were endemic species: Hucho hucho, Rutilus pigus, Gobio albipinnatus, Gymnocephalus schraetser, Zingel zingel and Zingel streber.

Upstream migration
The dimensioning criteria for the upstream migration facility, which was built in 2015, were the fish species Barbus barbus and Hucho hucho. The fish pass consists of a rough channel for the downstream connection at the entry of the fish pass. It is located on the right-hand side 300 m downstream of the power plant. An existing ditch has been used to facilitate the fish passage over 4.5 km and for providing new hydromorphological structures and habitats in the fish pass. Upstream, a vertical slot connects the nature-like fish pass to the river. This migration facility is always supplied with a minimum flow of 450 l/s which can be extended to up to 800 l/s.

Continuity high
Hydrology moderate
Morphology moderate
Agriculture significant
Morphology moderate

Fish migration
Over 43 fish species
Mainly potamodromous species
Fish pass (4.5 km)

Research objectives and tasks

Within the project, the fish pass at the plant shall be evaluated regarding the effectiveness of its existing hydromorphological structures. Both, the variation of the original structures to the current status as well as the usage of the existing habitats by fish shall be evaluated. Based on the results a concept for improved hydromorphological measures shall be developed under the consideration of acceptance by fish and expenses for maintenance. Moreover, the experiences with the maintenance of the nature-like fish passes shall be evaluated. This will be relevant for further decisions on the implementation of upstream migration measures.

Research tasks
The research tasks and field studies conducted at the Altheim HPP are:

  • Mapping the availability of habitat structures and analysing their added value for migrating fish
  • Evaluating the maintenance costs of a nature-like fish pass

Main FIThydro partners involved in the research at the Altheim HPP

Research topics FIThydro partners
Evaluation of maintenance costs of fish pass Uniper
Mapping habitat structure availability & analysing added value for migrating fish Uniper

Research objectives
Maintenance costs
Habitat availability & use

Update and results

Over a total length of about 1.5 km of the natural fish pass 37 different habitat structures have been implemented. The structures mainly consist of dead wood and spawning gravel and were constructed as spawning habitats, juvenile habitats or shelter but also to generally increase the variability in flow conditions. The value of these structures was evaluated from October 2019 to May 2020.

To evaluate the potential ecological value of the measures, five habitat structures were selected and examined by using the underwater cameras „Rollei 525“. Three cameras were placed around the habitat structures and three were placed at the other, unaltered bank. This approach allowed to compare and assess the fish density between the two sides. Furthermore it was possible to compare the five habitat structures with each other on the basis of predefined parameters and to frame potential differences regarding fish density and fish species.

The results show that the habitat structures provide a significant ecological benefit. They offer habitats especially for small fish species such as schneider (Alburnoides bipunctatus) to support their population. Especially the variability between fast flowing areas and flat, flow-reduced sections support the availability of specific habitats for different species. Also it turned out that there are major differences between the habitat structures regarding fish density and the number of fish species. This is probably caused by seasonal variation and preferences, abiotic factors as well as the location and type of habitat structure.

The high velocity and cool water temperature in the fish pass generally support the establishment of a sustainable grayling population. This fact is especially important regarding the threatened status of the grayling and the lack of habitats in the main river Isar. It shows the high water quality and good habitat availability of the fish pass. The fish pass turned also out to be a suitable habitat for ground oriented fish species such as barbel gudgen, zingel or European bullhead.

The criteria for habitat structures adding value to the fish pass do highly vary with the species targeted at. Structures with good flow conditions and clean gravel substrate in combination with the low water temperature showed excellent results for the target species of river Isar. Also the position of the structure is important to avoid deposition of fine sediments and also over a variety of water depths.

The maintenance effort does mainly increase with the length of the fish pass but not with the implementation of the habitat structures. It is important that the fish pass contains the necessary maintenance infrastructure such as ways along to be used by cars and heavy machines to make sure the maintenance works can be done properly. The habitat structures are expected to need a proper refurbishment after a longer time period than analysed in within the project.

Adding of gravel in the fishway

Adding of gravel in the fishway

Underwater monitoring of new habitat structures shows use by fish

Underwater monitroing shows habitat usage by fish

Placing of deadwood

Placing of deadwood for habitat creation

Underwater monitoring shows fish using deadwood as habitat structures

picture rights: © UNIPER