CONCLUSIONS : The historical changes of the Danube system are a consequence of geological development and the often-changed climatic relations during Quaternary time. One has to include the changes in the volume and movement of gravel and fine sand in the Danube, a deepening, increasing and meandering of the riverbed, sedimentation and erosion, often floods. The river inundation has also been affected through vegetation changes, by the intensive felling of forests, and preparation of new agricultural land, intensive draining measures, and the construction of drainage and irrigation systems, and the river dikes. At the same time, the changes caused by urbanisation, industrialisation, population growth, transportation and communication systems development, transformation to a modern agriculture based on chemicals, as well as the overall chemical contamination, all have to be taken into consideration. Independent experts of the Commission of the European Communities in their working group report , on November 23, 1992, stated: "In the past, the measures taken for navigation constrained the possibilities for the development of the Danube and the flood-plain area. Assuming that navigation will no longer use the main river over a length of 40 km, a unique situation has arisen. Supported by technical measures, the river and flood-plain can develop more naturally".
Danube, Danubian Lowland, Geology, Hydroelectric power project
Danube, Donau, Dunaj, Duna, … a poetic river, reappearing in its untouched shape, accompanied by the nostalgic melody of Johann Strauss’s On the Beautiful Blue Danube waltz. The natural evolution of the Danube and the changes resulting from a dynamic development of civilisation, along its banks contributed to the present character of the Danube, which seems to be as untouched as European nature in general (Fig. 1).
The historical changes of the Danube system are a consequence of geological development and the often changed climatic relations during Quaternary time. One has to include the changes in the volume and movement of gravel and fine sand in the Danube, a deepening, increasing and meandering of the riverbed, sedimentation and erosion, and frequent floods. The nature has also been affected through intensive felling of forests, preparation of new agricultural land, intensive draining measures, and the construction off irrigation systems and river dikes. At the same time, the changes caused by urbanisation, industrialisation, population growth, transportation and communication systems development, transformation to a modern agriculture based on chemicals, as well as the overall chemical contamination, all have to be taken into consideration.
It is beyond question that the current condition of the Danube and its flood-plain is the result of centuries of human intervention. It is a river that has contributed greatly to the development of the States sharing the Danube basin. It is a river that has been extensively utilised for navigation, water supply, fishing and more recently for hydroelectric power production and other purposes. I is equally beyond question that whenever measures are taken to modify the flow of a river, as contemplated by the Gabčíkovo - Nagymaros hydroelectric power project, there will be environmental effects, some adverse. This is true of all projects. The same modern technology that has made possible complex river projects has also led to techniques to measure the environmental impacts and to avoid, offset, mitigate, or remedy them. In the EC Fact Finding Mission report  it was concluded that “the environmental impacts of reducing the discharge in the Danube are negative, unless proper remedial actions are taken”. As will be shown below, such impacts were dealt with and with a great deal of success.
Independent EU experts  in November 23, 1992 outlined the state and trends in the area, “Before the 18th century the Danube split downstream from Bratislava into two almost identical arms. Near Bratislava it was partly a braided river with many small islands, as a result of progressive sedimentation where the Danube entered into the plain. Both arms were however meandering river systems and the Little Danube (Malý Dunaj) still is. Large changes occurred during the 19th century, when the first regulation works started. Within several decades the system changed into a braided river. Some of the older branches are still present in the landscape”.
“With the past endikements, especially during the last century, flood peaks became steeper and higher, flooding more frequent but in general with a shorter duration. The original zoning in vegetation towards higher grounds and associated forests was largely ‘diked’ out of the system. Most of the higher, no longer flooded soils, were converted into agricultural lands” .
“These river regulation works led to a deliberate and natural cutting off and bundling of river branches into one main, straightened and heavily fortified channel for navigation. This remaining channel is characterised by rapid water level fluctuations and very large stream velocities. The cut off branches, behind the fortified river banks, are only activated at higher discharges. Within the river branches many small weirs and dams were built, so most of them behave like cascade systems at low discharges. The interaction with the side arms so created became limited.” According to the experts of the Commission of the European Communities , flow in almost all river arms in pre-dam condition existed on an average of only 17 days per year, see Fig. 2.
The Gabčíkovo - Nagymaros Project consists of two parts, or steps, the Gabčíkovo part of the Project and the Nagymaros part of the Project. The Gabčíkovo part of the Project is situated in the central part of an intermountain depression, the Danube basin, called in Slovakia "Podunajská nížina" (Danubian Lowland). The Danube basin is filled by Late Tertiary (marine and lacustrine sand, fine sand, clay, sandstone, shales) and Quaternary sediments (river Danube sand and gravel settled in fluvial or lacustrine conditions). The total depth of the Quaternary and Tertiary sediments is 8000 m, with the uppermost Danube River sediments creating a main aquifer of high permeable gravel and sand. The thickness of the river Danube sediments, or the Danubian aquifer, ranges from a few metres at Bratislava to more than 450 m at Gabčíkovo, and goes back to a few metres downstream of Sap in the direction towards Komárno. Beneath this, a system of substantially less permeable aquifers and aquitards exist.
The important factors in the Danube transport and sedimentation processes are the existence of a granite threshold connecting the Alps and the Carpathians in the area of Bratislava, with an outcrop of granites in the Danube River bed. A similar hard rock river threshold, predominantly of andesite rock is situated at Nagymaros (between the cities of Štúrovo-Estergom and Visegrád- Nagymaros), some 160 km downstream from Bratislava. Both thresholds are natural geological hydraulic barriers, steps or thresholds, in the river bed. These are the upstream and downstream geological boundaries of the aquifers and the hydrological barriers naturally damming the Danube River bottom (Fig. 3).
Typical for such thresholds are a high gradient of the riverbed, high water-flow velocities and therefore lower navigation water depth, higher erosion downstream of such a threshold, moving fords, meandering of river and river arms, etc. The part of the river at Bratislava, just downstream from such a threshold, is a typical example. The flow velocity is high, the aquifer is shallow but with an extremely high hydraulic conductivity (permeability). Two municipal waterworks are situated at the granite threshold one on each side of the river. The Bratislava waterworks is on the Danube left side, Sihoť island, and is more than 100 years old. The second waterworks at Pečniansky les is on the Danube right side. These waterworks supply Bratislava with drinking water of some 1500 and 600 l/s, respectively. Both river banks in front of these waterworks are natural. And this is the place where the impact of the Gabčíkovo step starts, with a slight increase of the Danube water level.
Just downstream from Bratislava the Danube forms two branches, the Malý Danube in Slovakia and the Mosoni Danube in Hungary. These branches create two analogous islands, "Žitný ostrov" in Slovakia and "Szigetkőz" in Hungary. In the Gabčíkovo part of the Project, between Bratislava and Medveďov, the Danube formed an "inland delta" region, in geological literature expressed as an alluvial fan, through which it once meandered. This “inland delta” has its original typical morphology, i.e. meandering river, coarse sediment accumulation and erosion, changes in river bed gradient, etc. This large alluvial fan consists of an highly permeable extensive aquifer, capable of carrying and transferring high volumes of ground water. The Danube flows on the top of this “fan”, see Fig. 4. Water from the Danube infiltrates into the fan sediments and flows downward as ground water through the Danubian Lowland, nearly in parallel with the Danube river. In the lower part, where the slope of the river and the surrounding area suddenly decrease to the one quarter of its gradient at Bratislava, the ground water flows back into the Danube river via its own river arms, the Danube tributaries, and the drainage canals (Fig. 5). All this occurs because of the lowered permeability, and lowered aquifer thickness downstream from Gabčíkovo, which is a result of changed sedimentation conditions upstream of the andesite hard rock threshold barrier at Nagymaros.
The hard rock granite threshold and the andesite threshold, which naturally dam the Danube river bottom, and the places where the alluvial fan ends (a sudden decrease of river gradient from 40 to 10 cm per kilometre) are also important from the viewpoint of decision making. At these places there have been proposals to situate the hydropower stations known as Wolfstal, Nagymaros, and Gabčíkovo, respectively (Fig. 3).
According to the mutually agreed plan and Treaty 1977  between Hungary and Slovakia, the Gabčíkovo-Nagymaros project is hydrologically connected to the previously planned Slovak - Austrian hydroelectric power plant at Wolfsthal, upstream from Bratislava, and to the project Adony, downstream in Hungary (river kilometre - rkm 1601). The technical proposal is in accordance with the concept of the Rhine-Main-Danube and Danube-Oder-Elbe navigation system and with all hydropower stations and dams on the Danube.
In the German sector of the Danube, some 26 hydroelectric power projects have been completed. In Austria, ten hydroelectric power plants with navigational locks are in operation on the Danube. A chart listing these Austrian plants and the year of construction appears bellow.
Tab. 1.1. List of Austrian hydroelectric power plants on the Danube
The Gabčíkovo part of the Gabčíkovo - Nagymaros project
The Gabčíkovo part of the hydroelectric power project Gabčíkovo-Nagymaros was based on a combination of flood control, navigational improvements, production of electrical energy and protection of nature. In their working group report  independent experts of the Commission of the European Communities, stated on November 23, 1992: "In the past, the measures taken for navigation constrained the possibilities for the development of the Danube and the flood-plain area. Assuming that navigation will no longer use the main river over a length of 40 km, a unique situation has arisen. Supported by technical measures, the river and flood-plain can develop more naturally".
It emerges from the report of the Commission of the European Communities tripartite fact-finding mission , dated 31 October 1992, that “not using the system would have led to considerable financial losses, and that it could have given rise to serious problems for the environment”.
The main structures of the Gabčíkovo STEP
The hydroelectric power station (Fig. 6), consisting of four blocks in which eight turbines and generators have been installed. They are all vertical Kaplan turbines, with runners 9.3 m in diameter and a maximum capacity of 90 MW each. The total installed capacity of the hydropower station is 720 MW with an operational discharge of 4000 m3/s. Minimal and maximal discharges are 413 and 636 m3/s per turbine, inversely related to water level differences of 24.0 and 12.88 m, respectively.
Two navigation locks serve passing ships and barges sailing along the Danube. Each lock is 275 m long and 34 m wide. The difference in water levels between the upstream and downstream canal varies from 16 to 23.3 m.
The bypass canal, consist of the headwater section upstream from the navigation locks, a hydroelectric power station, and a tail-race section (outlet canal) downstream from the power station.
The Čunovo reservoir is a part of the original Hrušov-Dunakiliti reservoir, which is situated exclusively on Slovak territory. The area of the originally designed Hrušov-Dunakiliti reservoir is 6000 hectares, and of the Čunovo reservoir approximately 4000 hectares, depending on water level. The operational water level at Čunovo is about 131.1 m a.s.l. (above the Baltic Sea level); the minimal and maximal operational levels are 129 and 131.5 m a.s.l., respectively. Ensured navigational depth is 3.5 m, according to requirement of the Danube Commission.
The intake structure at Dobrohošť supplies the inundation river branch system with water, it enables flood simulations for forestry and ecological purposes. The discharge capacity is up to 240 m3/s.
The original function of the Dunakiliti weir in the Gabčíkovo part of the Project is fully substituted by the Čunovo weir constructed on the Slovak territory and inside of the original reservoir area, upstream from the Dunakiliti weir.
Because at present the construction of the Nagymaros part of the Project on the Hungarian territory has not been built, the Gabčíkovo power station is operated as a run-of-the-river plant in a “water-level regime”, meaning that the head water level is fixed and the allowed water level fluctuation ± 4 cm at a low flow discharge of up to 1500 m3/s, and ± 15 cm at a higher flow discharge.
Ecologically and social important structures and areas
The main parts of the area and of the Gabčíkovo hydroelectric complex having ecological importance and importance to the regional development are shown in Fig. 6:
- Protected Landscape: CHKO Dunajský luh (Danube flood-plain), established on the May 1, 1998, as a response to the new hydrological conditions.
- Nature reserve localities: Ostrov Kopáč, Topoľove hony, Gajc, Hetméň, Jurovský les, Ostrovné lúčky.
- Protected sites: Bajdel, Poľovnícky les, Dolný hon, Park v Báči, Park v Rohovciach, Park v Kraľovičových Kračanoch, Park vo Vrakúni, Park v Gabčíkove.
- National nature reserves: Ostrov Orliaka morského, Číčovské mŕtve rameno.
- Nature monuments: Pánsky diel, Kráľovská lúka.
 CEC Nov. 23, 1992: Commission of the European Community, Czech and Slovak Federative Republic, Republic of Hungary 1992. Working Group of Independent Experts on Variant C of the Gabčíkovo-Nagymaros Project, Working Group Report, Budapest, Nov. 23, 1992
 FFM Oct. 31, 1992: Commission of the European Communities, Czech and Slovak Federative Republic, Republic of Hungary, Fact Finding Mission on Variant C of the Gabcikovo-Nagymaros Project, Mission Report, October 31,1992
 FNSCU, 1995:Gabčíkovo part of the Hydroelectric Power Project - Environmental Impact Review, collective volume, Sc. Editor I. Mucha, Faculty of Natural Sciences, Comenius University, p. 384.
 PHARE 1995: Danubian Lowland - Ground Water Model, Final report, Vol.: I, II. III, Ministry of the Environment, Slovak Republic, Commission of the European Communities
 TREATY 1977: Treaty between the Hungarian People´s Republic and the Czechoslovak Socialist Republic concerning the construction and operation of the Gabčíkovo-Nagymaros system of locks, Unated Nations Treaty Series, Vol. 1109,I. 17134
 Agreement 1995: Agreement between the Government of the Slovak Republic and the Government of Hungary about Certain Temporary Measures and Discharges to the Danube and Mosoni Danube, signed on April 19, 1995