VII.   Future of the Danube and its river branch system between Dobrohošť and Sap 

VII.1.  Potentials of the future Danube ecosystems

Mikuláš J. Lisický, Igor Mucha 

The ecosozological status of the Danube ecosystem and the adjacent floodplains declined proportionally to its anthropogenous changes, which occurred especially in the last 150 years. According to the international criteria, its state in the mid 19^th century would correspond to the category of World heritage. In the mid 20^th century it still had values corresponding to a national park. At the time just before the start of construction of the Gabčíkovo project it had the values only of a large-size protected area in the category of protected landscape. The Gabčíkovo project represents a bifurcation of the potential development trajectory. On one hand it emphasises anthropogenous limits in the broad area and a reduction of its functions for agricultural, navigation, energy, and silvicultural (commercial forestry) exploitations. On the other hand, it has opened a possibility of agreement on conditions of fewer conflicts over the spatial delimitation of its functions, and better possibilities for its integral water management, including flood protection. Although most development during the last 15 years indicates rather more realization of the first alternative, there still exists a possibility to rehabilite the ecosystem in the limited within-dike area with limited flow rates. These flow rates (into the Old Danube) are defined by the Slovak-Hungarian Agreement as follows: the annual average flow rate 400 m³.s^-1; the flow rate in vegetation season up to 600 m³.s^-1; the flow rate out of vegetation season at least 250 m³.s^-1. In addition, there were agreed flow rates into the Mosoni Danube arm, about 40 m³.s^-1 and other flow rates supplying the inundation area and agricultural area behind the protective dikes. Such flow rates and the corresponding water volume in the annual balance can be used ecologically more effectively when they are related to the natural seasonal dynamics of flow rate fluctuation in the Danube. It could be a preset flow (for example 3500 m³.s^-1 at the gauging station Devín), at whose exceeding the water management structures ensure that water in the arm systems would start to overflow the area between the Old Danube and flood protective dikes. Thus, “natural” flooding would progressively start. This is the basic idea, which has been elaborated and published in the form of scenarios and the final recommendation of an expert group [3]. We note that in the pre-dam conditions such a flow rate of 3500 m³.s^-1 was already not enough to overflow the area, neither from the Danube main channel nor from its side arms. The discharge of 3500 m³.s^-1 occurs on average about 52 days a year, while that of 4500 m³.s^-1 occurs about 17 days a year [4]. 

Number of environmentalists, and surprisingly even some ecologically educated specialists, are unable to interpret changes of the natural environment from the view of secular (centenarian) spatial and temporary changes, and consider environmental perception (opinion) from a time span of one or two ancestor generations as the reference state. As we have already mentioned in the chapters about changes of biota in the Danube inundation area, the extent of the Danube floodplain between the present-day Bratislava and Komárno was reduced in the past centuries in orders, while the flow rate through this area remained on a comparable level. In this way a gradually increasing disequilibria has arisen, which has unavoidably led to the concentration of flows into a narrow within-dike corridor. This has increased their destructive power and, on the principle of a chain reaction, called for further measures, first of all in flood-control and navigation. The structure of the wooded plant stand, which was gradually pushed by the forest management, especially after World War II, worsened the situation from the hydraulic viewpoint, notably at large flow rates. The natural forests in the floodplain along the Danube, as a large river in the Central European area, would not have such a large stem density, neither near the eupotamal (one main channel) nor along more parallel flowing channels. The Danube would not create favourable conditions for permanent transitional floodplain forests. On the contrary, such a large river would have created a permanent retention capacity in the form of wetlands and flooded meadows. The system mistake, which happened in the landscape care during the last two centuries, is a result of inadequate intergeneration memory about the dynamics of the Danube hydrological regime. 

Although any of the following comparisons may be not fully acceptable, we consider it to be purposeful to try to find out a parallel between anthropogenous and natural changes in the alluvial landscape. If we simultaneously draw attention to their common and different features, we can reach inspiring conclusions. In the case of the hydraulic structures built on a bypass canal, we can speak about an anthropogeneous avulsion (abrupt change) and the subsequent aggradation. A sudden change of the riverbed is natural; unnatural, however, is the transformation of its character into an isolated canal, which does not communicate with the adjacent water bodies. The elevation of the water level in the bypass canal can be compared with aggradation, which is here represented by lateral dams, without a corresponding elevation of the bottom altitude. Aggradation was a natural phenomenon in the Danube development, but it was accompanied by subsequent lateral erosion. The unnaturalness of the bypass canal lays in the minimal permeability of its riverbed and its permanent straightening. The character of its banks is degraded from the biological viewpoint. It does not offer conditions for organisms usually living in riverbanks; the canal bank does not enable infiltration and refiltration, and does not represent a habitat for interstitial fauna. The river energy is under natural conditions used diffusively, but from the ecosystem viewpoint “in situ” for landscape forming processes. The river energy concentrated by the power station and led out of the space, is used “ex situ”, for economic purposes, which are strange to the ecosystem. If we enlarge such an energetic balance to the whole area, which was directly influenced by the river in the past, we see that a part of the misappropriated energy after transformation (for example on agrotechnical measures, supply of nutrients) returns into the landscape. This energy is used in favour of xenocoenous species (field crops and animals bound to them). Such a floodplain is not expected to fill its original roles. It became alienated also functionally misappropriated. However the river, which formed this floodplain in the past, is expected to provide a favourable regime of ground water levels. Up to this point it is logical and no essential problems rise from the viewpoint of equilibrium. On the contrary, such problems rise in the within-dike area, especially when it should be used economically. 

Unlike ecosystems, human technologies have, up to the present, a limited ability to use diffused energy (see the existing problems in getting solar or wind energy). Therefore they are based on concentration of their resources, point transformation, and redistribution. It is, of course, connected with many problems of a logistic character, which are usually solved at the expense of natural environment quality. Similarly, the existing adaptability of agriculture is not so perfect to be able to use without conflicts areas of wetlands with shallow water (a specific exception is the cultivation of rice). Therefore anywhere agriculture comes with its interests, it tends to redistribute the natural continuum into entities serving partial interests and to give them discretely economically exploitable functions. Even delimitation of the landscape into the economically used areas and protected areas (nature reserves), supported by the classical nature conservation, was based on this same principle. At the beginning, such delimitation more or less worked, because the protection referred only to small economically uninteresting areas. From the time that the institutionalised nature conservation began to push the principle of large-size protected areas, while recreation and sport have become a perspective economic exploitation of another kind (sui generis - unique), the conflict comes back. 

The principal precondition of success of all systemic measures is taking into account that it is not possible to optimise all parameters of the system. The optimum of a working system is not a sum of optimum states of all its components, but their optimal interplay. It has its time-space dynamics. In the case of an ecosystem it means that ideal conditions for some ecological groups are provided in one time span, while the ideal conditions for other groups in another period. Therefore, it is necessary to exclude an a priori attempt at harmonization of all demands for optimality as defined by specialists from the viewpoint of one group. As an example we can present the attempt to set a norm for optimal simulation of floods. 

From the viewpoint of optimising conditions for fish, it is necessary to provide each year one flood with cold water and one flood with warm water. Both floods should have sufficient duration, and it would be best if the whole within-dike zone would be flooded. The first spring flood should start in February, culminate in March and decay in April. The second summer flood should start in May, culminate in July and decay in September (see Černý in this volume). 

From the viewpoint of development of plankton, which is important for many fish species and food, it is optimal if the flow in an arm system is limited and slow and floods are not too frequent and not too dynamic. From the viewpoint of poplar plantations it is optimal if the surface flood does not occur at all and the river functions similarly to in agricultural ones in the out-of-dike area, are restricted to arrange for an optimal ground water level. This should be higher in the first part of vegetation season and lower at its end. On the contrary, from the viewpoint of inhibition of allochtonous species, especially of invasion herbs, it is optimal if a flood lasting several weeks occurs at least once a year. From the viewpoint of organisms that naturally live in the floodplain, but are not able to survive a several weeks long flood (for example edafon), it is optimal if the flood is followed by a longer period without floods, during which the connectivity on the river enables passive migration of living individuals for rehabilitation of populations. Such and similarly optimal floods could occur in this area as late as after building up of the dikes, they could not be typical of the original natural alluvium. Hence we are speaking about criteria of adaptive optimality. The problem is that in a much smaller area it is necessary to provide not only the original flow capacity, but also the original diversity of habitats and species composition. If we have to define, in spite of this, the optimal ecological state, it will be that, which most converges to the natural state (not to the adaptively natural one). 

Hydraulic structures with the bypass canal, placed out of the within-dike zone, paradoxically offers the possibility not to norm all criteria, which must be filled from an ecological viewpoint by the within-dike area, but to provide them by a natural flow rate matching to its size (extent of the inundation area was manifold reduced during the past centuries) and to restore, in this way, autoregulation of the naturalness and its selection in the within-dike zone. But it cannot be concealed that this means an essential change in forest management, and the limitation of recreation to a degree that does not conflict with the environment naturalness. Paradoxically, at the same time the Gabčíkovo structures are offering new possibilities for water sport and recreation at the area of the reservoir and at the Old riverbed reserved for leading away flood discharges. The landscape and flow of the river, which forms it, can again return to equilibrium after a well considered intervention. 

These considerations have led the authors [3] to set questions of priorities in the inundation area, and to comment on different scenarios of the possible future development of a natural environment in the within-dike area between Rajka and Sap from this viewpoint. The extent of this publication does not allow repeating comments of individual scenarios. In this regard we refer to the published monograph [3]. Here we discuss just a brief characteristic of the scenario, which appeared, under conditions of respecting the existing limitations (size of within-dike zone, disposable flow rates for its rehabilitation, and preservation of its flood control function towards the out-of-dike area), as the ecologically and ecosozologically most suitable. Simultaneously it sufficiently respects the flood control function and existing limitations. 

We consider the so-called Old riverbed of the Danube to be ecologically dysfunctional and, from the viewpoint of heavy navigation, needless. Therefore, it can be abandoned so far as concerns the discharging of water. However, it cannot be filled or successively overgrown by floodplain forests because of its flood control role. Instead, it must be maintained without cost consuming technologies (removal of growths, dredging, etc.) as a corridor able to lead discharges exceeding the total aggregative capacity of the bypass canal, the rehabilitated river arm systems and floodplain forests on both riversides. It is necessary to restore the integrity of the ecosystem, which split after finishing the Gabčíkovo hydraulic structures into three relatively autonomous systems (the abandoned riverbed and two arm systems artificially supplied with water). The simplest method, how to solve it, would be damming of the whole stretch of arms by several weirs. However, we do not consider such a mode as ecologically favourable, because it inserts lentic stretches (slowly flowing lakes) into a markedly flowing water (lotic) ecosystem. Under conditions of reduced discharge, the arm systems of inundation have to take over the functions of a bearing system of an anastomosing eupotamal (branching and rejoining irregularly to produce a net-like pattern). This pattern existed in the Danube in the remote past on a much larger scale. It can be presumed that in the changed condition, in the within dike inundation, it could represent its contracted model. Reconnecting of the two arm systems should be enabled. Models must verify concrete places and flowing conditions. It can be presumed that it will be the so called gates, where the former main stream recently revives communication with side arms via previously closed entrances during flood (for example in 2002). The water flowing from the present left side inundation area into the right side area and vice versa must have an expressively flowing character and potential for lateral erosion. In the places where it is not excluded because of the security of the existing flood protecting dikes, the natural erosion should be allowed to remodel the present terrain shape. In this way, after removal of the existing weirs, the bedload regime of the river will be restored in this area. The bearing discharging system must be adjusted to enable not only the riverbed-forming processes but also ensure the dynamics of water level fluctuations in those side water bodies, which will not be permanently connected with this system. 

The principle of this proposal is to lead the flowing Danube water from one side arm system into the other. During its crossing of the old riverbed, the water shouldn’t lose its expressively lotic character. Thus this solution cannot be based only on connection of the arm systems with backwater in the old riverbed. Such a solution would mean that the lotic fauna of the eupotamal would drift into a lenitic environment, while the lenitic fauna would drift into the streaming water of the arms. If an agreement about crossing of the old riverbed will not be reached, there could exist two parallel arm systems. But in this case it would be ecologically worthless or even undesirable to connect them with the backwater ecosystem in the Old Danube. 

The most serious tasks of this project, whose solution must be based on modelling, will create a solution for crossing the old riverbed so that water flowing between the arms across the Old Danube is not lost in the old riverbed. It will be necessary to build up a continuation of new eupotamal riverbank lines across the Old riverbed with boulder-chute, and to propose the shape of these weirs so the floodwater can be lead through the old riverbed, or a critical flood is able to take them apart. 

When comparing the similarity of the restored ecosystem with its natural pattern as it existed there in the remote past, several through flowing “reservoirs” appear in the old riverbed as added non-original elements. However, under the presumed hydrological regime, they will have a semi natural character and will not substantially influence the restoration of natural qualities of the ecosystem. On the contrary, they will even increase its diversity. 

It is natural that the 15-year experience with the behaviour of the river and its natural environment after putting the Gabčíkovo hydraulic structures into operation leads the ecologically oriented specialists to convergent concepts, prognoses, and proposals of repairing measures. They can differ in the proposed intensity, but they will agree in the opinion that the existing trend is undesirable from the nature protection viewpoint as well as from the viewpoint of flood control. Two years ago we had the opportunity to get aquainted with similar idea of the Hungarian specialists [5] and we embrace the possibility of common policy in solving the common problem. It is necessary to find a common strategy as soon as possible and to begin to act.  

References