Hidrológiai Közlöny, 2016 (96. évfolyam)
2016 / 3. szám - HISTORICAL SNAPSHOT - Borics Gábor - Ács Éva - Boda Pál - Boros Emil - Erős Tibor - Grigorszky István - Kiss Keve Tihamér - Lengyel Szabolcs - Reskóné Nagy Mária - Somogyi Boglárka - Vörös Lajos: Water bodies in Hungary - an overview of their management and present state
G Borics et al: Water bodies in Hungary - an overview of their management and present state 59 flora and fauna causing ecosystem-level changes and undesirable consequences. Tisza River The Tisza takes its source in the Eastern Carpathians (Ukraine) crosses the Great Hungarian Plane and finishing its 962 km long route enters the Danube in Serbia. Development of the present state of the Tisza valley can be dated back to the mid-19lh century when comprehensive regulation of the river started. The originally 1419 km long river has been shortened, embanked, and more than 20,000 km2 area became protected from floods. Besides the positive impacts of this huge engineering work negative outcomes also occurred. Drying out of the valley threatened the safety of the agricultural production and became an urgent problem by the middle of the 20th century. This necessitated the building of two river barrages (Tiszalök 1959; Kisköre 1974) and several canals in the middle part of the valley. Although the danger of annually arriving devastating floods has been greatly reduced, deforestation of the upper catchment and the extreme events of precipitation in the Carpathian Basin contributed to the development of extreme floods and low discharge periods in the recent years {Fig. 5). Figure 3. Forest on the Tisza floodplain (Photo: Béla Csányi) These new challenges required the rethinking of previous measures and called into being new strategies in river management. Several off-river storage reservoirs have been planned and established in the middle Tisza valley to reduce floods and to store water in dry periods. Despite the engineering works substantially altered the landscape in the Tisza valley the river and its immediate surroundings managed to preserve their natural character. Although the large wetlands that formerly characterised the Tisza valley have been drained, many water related natural and semi-natural areas have been preserved, restored or established newly, which act as green corridors. Due to urbanization, industrial development and the intensification of agriculture water quality of the Tisza and its tributary rivers considerably worsened from the middle of the last century. The untreated sewage effluents caused drastic organic and (after their degradation) nutrient load, which led to the eutrophication of the rivers with its all negative consequences. This negative tendency changed from the 1990-ties when several factories were closed and hundreds of sewage treatment plants were established in the region. The water quality and ecological state of the river considerably improved. The enhanced recovery potential of the Tisza river is clearly indicated by the quick recovery of the biota after the serious cyanide pollution that occurred after an industrial accident in Romania in 2000. Small streams During the comprehensive regulation of rivers in the Tisza valley the flooded areas in plains were almost totally eliminated. The landscape has been affected by the digging of ditches and the drainage of wetlands for agriculture. Lowland streams have been straightened, deepened and widened to facilitate land drainage and to prevent local floods. Consequently, the trees in the shoreline were diminished, buffer zones were eliminated and the level of groundwater decreased. These small streams are vulnerable to changes that anyway have little effect on larger water bodies. They have sufficiently large catchment area to be adversely affected by human impacts. Small rivers with a small volume of water have only a limited ability to dilute and retain pollution, and therefore they are highly susceptible to inputs of pollutants from their surroundings, such as nutrients and pesticides from agriculture. Excessive sediment movement caused by erosion of streambed is also a factor affecting the ecological status of water bodies, particularly in small lowland streams. In addition, dry periods and water abstractions can greatly reduce their water flow and water level. Therefore, these small streams generally have poor ecological quality. There are, however some parts of the Tisza valley where the small lowland streams were less affected by human impacts (Fig. 4). The streams which show the features of former natural ones represent unique and important ecological systems. They support specific and important hydrological, chemical and biological processes and provide proper habitats for a wide spectrum of plants and animals. However those few small lowland streams that are still in good ecological state are also exceptionally vulnerable to climate change impacts. The increasing water temperature and the likelihood of water scarcity and droughts can seriously affect the ecological status of small streams. Figure 4. Small lowland stream in natural state Unfortunately, achieving of the good ecological status of the small lowland streams in Hungary, among others, is impeded by the low stream gradient (slope of the
