The Flow of the Danube
The Danube is a fast-flowing, turbulent river. Its flow ranges from 400 to 10,000 cubic metres per second, and averages at around 2000 cubic metres per second.
Just a few stretches of the Danube can still be characterised as free-flowing. The upper part of the Danube has been ideal for building hydropower plants due to a rapid current of between 8 and 9 km/hour caused by the river bed's natural gradient. In recent decades a total of 59 dams have been built along the river's first 1,000 kilometres, an impressive average of one dam for every 16 kilometres.
The largest hydropower dam and reservoir system along the entire Danube is located at the 117 kilometre-long Djerdap Gorge where the river forms the boundary between Romania and Serbia. The system consists of two massive dams, the Iron Gate Dam I and II, measuring some 300 by over 30 metres. These dams arejointly operated by the two countries. The Iron Gate system has a flow rate of 5,500 m3/second and the river drops 34 metres while passing through. The dams trap some 20 million tonnes of sediment and pollutants per year. Downstream of the Iron Gate, the river is free flowing all the way to the Black Sea, a distance of more than 860 kilometres.
The second largest dam system is operated at Gabcikovo, downstream of Bratislava, where 90% of the Danube's flow is channelled towards a power plant to generate some 10% of the electricity used in Slovakia.
Water Quality of the Danube
The Danube is a good drinking water source in many locations. In Germany, almost thirty percent of the water for the area between Stuttgart and Alb-Donau comes from purified water of the Danube. Other cities like Ulm and Passau also use water from the Danube. Another ten million people get their water primarily from groundwater through domestic wells whose source is the Danube. Although good drinking water quality without treatment has only been achieved in some stretches of the Danube, its oxygen levels are high enough to allow treatment with natural processes, such as bank-filtering or sand filtration to reach drinking water quality.
Overall water quality has improved during the last decade, but further improvement is still needed. Most stretches of the Danube can be described as moderately polluted, but some tributaries and stretches of the lower Danube fail to achieve this status. Harmful substances from farmland and heavy industries pollute the rivers and undermine the quality of the water. Also, most of the Danube's tributaries contain higher concentrations of organic pollutants and nutrients than the Danube itself.
In order to improve water quality, countries up and downstream of the polluted areas are working together to clean up river banks, impose stricter rules for industries, conceive wastewater treatment plans and return the riverbed to its natural state to create wetlands and prevent floods and droughts.