4. River branches area

Independent experts of the Commission of the European Communities in their working group report stated [1]: "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."

The Danube River branch system, with its typical alluvial inundation, is an area between flood-protecting dikes and the main river (Fig. 4.1?4.4 and Fig. 4.5). This area was not touched by the construction of the by-pass canal, hydropower station or other engineering structures. On the Danube this is a really unique situation. In general, two basic ideas have been implemented. The first idea, to supply water to the inundation river branch area, hasbeen realised on both sides of the river. On the Slovak side the intake structure is in the upper part of the inundation area at Dobrohošť; on the Hungarian side the intake structure is constructed directly inside the Dunakiliti weir, in the upper part of Szigetköz. The second idea was to recover the lowered water level in the Danube, which had declined in the past, and then further, after putting the Gabčíkovo Project structures into operation. Various solutions have been proposed, all of them based on some structures in the original Danube. One of the accepted versions are shallow underwater weirs together with existing navigation spur-dikes between them. Some other, similar measures are the stabilisation of natural fords, low weirs(for example, inflatable), ones creating river meandering, etc.

An intake structure to supply water to the Danube side arms on Slovak territory takes water from the bypass canal at Dobrohošť. Its discharge capacity is up to 240 m³/s. The Dobrohošť intake structure supplies the inundation area and river branch system with water and simulates water level fluctuation and floods for forestry and ecological purposes, e.g. the period needed for laying fish eggs, simulating floods, cleaning the river branches bed, etc. Water flow and water levels inside of the inundation area are regulated according to the needs of forest management by cascades with culverts and water passes. The first artificial flooding was realised in the period from July 19. to August 18, 1995. Hydrological analysis showed that the extent and duration of the flooding corresponded to the discharges in the Danube in pre-dam conditions of 3000 to 4500 m³/s.

A continuous water flow (or arbitrarily regulated water flow) into the river branches is guaranteed, areas covered by water and river branch banks are enlarged, the water level is higher, and water quality is dramatically improved. Conditions in the river branches resemble conditions in the 60's, before heavy stabilisation of the Danube River banks (done to improve conditions of navigation). This stabilisation of river banks had already changed the natural relationship between the river and the flood-plain. At present, there is a large variability of flow velocity and depth in the river branch system, with velocities at some places from as high as 1 m/s, to nearly stagnant water in side branches and stagnant water in old dead branches supplied by ground water. The settling of aquatic vegetation in shallow river arms creates suitable living conditions for a wide range of species. Thus, a large variety of living conditions was recovered. Water vegetation typical for areas having periodic desiccation has, with former high water branches, moved to places covered by simulated floods. An overall increase in primary and secondary aquatic production has occurred. The water is of good quality. The danger of anaerobic conditions (known from pre-dam conditions) and the drying up of branches accompanied by the extinction of fish, does not exist any more. Eutrophication, as far as it may occur in dead-end branches, will be within the range of original natural variation.

According to the international agreement on wetlands, the "Ramsar agreement", wetlands are especially important for waterfowl, and they are especially valuable areas because of their biodiversity. The term wetland includes bogs, marshes, swamps, morasses, moors, peat, natural and artificial water bodies, permanent and temporary waters, flowing and stagnant waters, fresh, brackish and salty waters, including the areas of sea water with a depth during an ebb (ebb- end tide) not larger than 6 meters.

According to the Ramsar agreement (Article 4), concerning all water bodies enlarged and created by the Gabčíkovo structures, and the increase of soil moisture as a result of surface and ground water level increases, whether they are included or not on the list of internationally important wetlands, the country should support the existence of wetlands.
Construction of the Gabčíkovo hydropower structures and hydraulic measures have increased the area of all types of water bodies, including all types of river branches, and have increased soil moisture of the inundation and river branch areas. The whole inundation area and flood-plain forests at Gabčíkovo have been proposed as additions to the list of international important wetlands.

Due to the low water levels in the Danube, a water supply for the river branch system was created. This, together with a system of cascade hydraulic structures, hydraulic lines (named alphabetically from A to J), constructed nearly perpendicular to the main flow in the branches, help to keep the inundation area wet and can even be used for simulating floods (Fig. 4.6). The cascade hydraulic structures are constructed with culverts including water level regulating enclosure sets and water passes. The closure of culverts for a particular time period can increase the water level in the compartment behind the cascade hydraulic line, and thus simulate a flood wave or water level fluctuation. Such water discharges and the managing of the water level regime, and thus the optimisation of the whole environmental impact, is based on careful monitoring and its proper interpretation.