Since the 16th century, people have been changing the natural course of the rivers in the Danube River Basin, mainly for hydropower generation, flood defence and navigation. All these changes affect the ecological quality (technically also known as 'status') of the rivers.
Hydromorphological alterations such as river and habitat interruptions, the disconnection of wetlands/floodplains and hydrological alterations (i.e. water abstraction) can provoke changes in the natural structure of rivers. This may include the alteration of river depth an width, river type specific flow regimes, interruption of natural sediment transportation as well as the interruption of natural fish migration routes.
Large dams and weirs, for instance, might not only interrupt the river and habitat continuity, but have an important effect on the natural sediment transportation, resulting in
- the retention of sediment upstream of dams. The accumulated sediment has to be extracted to maintain the river's depth for hydropower generation and navigation. The siltation process can also entail problems with the drinking water supply.
- the loss of sediment downstream of dams, meaning that material must be artificially imported to stabilise the river bed and prevent incision.
Hydrological engineering works include dams, dykes, reservoirs, navigation channels and irrigation networks. Dams and reservoirs have been built in nearly all mountainous areas of the Danube Basin and in some lowland regions; navigation channels, dykes and irrigation networks are widespread in the lowlands along the middle and lower reaches of the Danube.
Plans for new dams
There are still plans for more dams to be built, for instance on the Bavarian Danube, on the Sava and on the Drava along the Croatian-Hungarian border, where the Novo Virje dam (planned capacity: 121 MW) would break up the still largely pristine 370-km stretch of river along the Mura and Drava between the Austrian border and the Danube.
The current status of the hydromorphological alterations (e.g. river and habitat interruptions, disconnection of wetlands/floodplains and hydrological alterations) as well as future infrastructure projects are investigated and addressed in the frame of the Danube River Basin Management Plan (DRBM Plan).
The most serious problems resulting from the construction of hydropower facilities are the disruption of the longitudinal continuity of the rivers, and dramatic changes in the rivers' hydrological characteristics.
Hydropower accounts for 30% of national generation capacity in Romania, which has more than 400 large dams. Around 60% of the yearly electricity generation within the Danube River Basin in Austria originates from hydropower.
Flood defence measures
Most large rivers in densely populated areas have been modified to help prevent floods, and to obtain new land for urban development. In many cases, hydrological engineering structures change river courses and channels considerably, with meanders and branches straightened and redirected.
The Danube itself is regulated along over 80% of its length. By cutting off the river from its floodplains, the frequency and duration of floods is changed, and former floodplains are degraded in ecological terms.
Dyke systems have been built to prevent floods along the Danube ever since the 16th century. Only about a fifth of the floodplains that lay along the Danube in the 19th century still remain.
Vast areas of floodplain have been affected by river regulation or flood defence measures - in Hungary for instance an area of no less than 3.7 million hectares has been dyked.
In the Danube Delta, embankments were built in an area of more than 100,000 hectares, mostly areas liable to temporary flooding, although natural conditions have been restored over about in 15% of this area by ecological restoration schemes carried out since 1994.
Large dykes and cross-cutting meanders and river branches also influence groundwater levels by suppressing the exchange of water between the rivers and the groundwater reserves. This affects the renewal of riverbank filtrate used to purify the drinking water supply in certain areas.
Large dam systems
The upper part of the Danube has been ideal for building hydropower plants due to the river's considerable natural gradient. A total of 59 dams have been built along the river's first 1,000 kilometres - from the source down to Gabcikovo - many of them decades ago. This means that the Upper Danube is interrupted every 16 km on average. Very few stretches can still be characterised as free-flowing.
Downstream of Bratislava, three more hydropower plants interrupt the flow of the river.
The largest hydropower dam and reservoir system along the entire Danube is located at the 117-km-long Djerdap (Iron Gate Dam I and II) Gorge. This peak operation system consists of two dams, jointly operated by Romania and Serbia. The average flow rate of the Danube here is 5,500 m³/sec, and the river drops a total of over 34 metres. The capacity of the hydropower facilities is 2,532 MW, and annual production amounts to 13,140 GWh.
The Iron Gate system has transboundary effects. The reservoirs have a total volume of 3.2 billion m³, and a total length of 270 km. They trap some 20 million tonnes of sediment per year, serving both as an important nutrient sink and a sizeable deposit of hazardous and toxic pollutants originating upstream of the dam. The corresponding absence of natural sediments downstream has created erosion problems since the dam was put into operation in 1970.
The Iron Gate Dams I and II, both located where the Danube forms the boundary between Romania and Serbia, impound the Danube as far upstream as Novi Sad. Major impacts include the interruption of river and habitat continuity and hinders therefore fish migration. Furthermore, considerable reductions in flow velocity and sediment transportation, and raised groundwater tables in the Serbian lowlands.
The second largest dam system is operated at Gabcikovo, downstream of Bratislava, since 1992. The dam is located near an area that used to form one of the region's largest wetlands. Today, 8,000 ha of floodplain forests and an extensive network of tributary channels are artificially irrigated. The original river channel nowadays: only receives 10-20% of the total flow, as the other 80-90% is channelled around the dam towards the power plant, where it is used to generate some 10% of the electricity used in Slovakia.
A 40-kilometre stretch of the original channel is badly affected by lack of sufficient water discharge. In some areas surface water and groundwater levels have dropped by up to 4 metres, greatly affecting the ecological conditions in the area's wetlands.
Restoration of river and habitat continuity
Migratory fish species, such as the sturgeon are good indicator of the ecological condition of the entire Danube River Basin. The Danube itself as well as the tributaries are a key migration routes. The Iron Gate Dams I & II, in part the Gabcikovo Dam, and the chains of hydropower plants in Austria and Germany represent migration barriers for fish. Migratory fish, such as sturgeon and medium distance migrators, are particularly affected, being unable to move up or downstream between their spawning grounds and areas used at other times in their life cycle.
In particular, the impact of the Iron Gate Dams I and II has resulted in sharp declines in most Danube sturgeon species, with significant regional economic impacts on the productivity of fisheries. As a response, the ICPDR has started an initiative to restore river and habitat continuity. This approach is explained in further detail in the Danube River Basin Management Plan.
- Map 5: River and habitat continuity interruption – current situation (2009) (1.07 MB)
- Map 27: River and habitat continuity interruptions 2015 (expected improvements) (1.08 MB)
- Map 13: Heavily modified and artificial water bodies (1.09 MB)
- Measures for ensuring fish migration at transverse structures (1.03 MB)