Summary:

The Water Framework Directive (2000/60/EC), implemented in Poland, obliges to improve rivers ecological state. A change of fluvial system management methods, consisting in reduction of technical means usage at channel maintenance, is necessary. The channels of mountain rivers and streams are especially interference-sensitive. Wrong river training works in these channels lead in the short time to drastic changes along them. The consequences of these changes are not only environmental, but also economic. This study demonstrates the present state of knowledge about the reasons of Carpathian channel changes, channel response to individual interferences, and modern alternative solutions in the matter of channel management. A method of comprehensive geomorphologic investigations of mountain channels is also presented. According to the opinion of the authors these investigations should be carried out in order to better recognize functioning of these channels. The main purpose of the work is to show the results of some activities which seems to be wrong in present time. The necessity of interdisciplinary approach to fluvial system management is accented because determination of adequate works improving environmental conditions of rivers requires research of correlation between many environmental components. Therefore this work is addressed to representatives of different disciplines, mainly to civil engineers, geomorphologists, biologists and ecologists. The evident acceleration of changes in mountain channels morphology took place during the last century. The channels changed their courses from braided to single-thread, straight or sinuous and were downcut from 1.0 m to 4.0 m. In order to recognize channel structure and dynamics as well as channel changes, a research method is proposed in the paper, which consists in mapping basic channel reaches (defined on the basis of the channel pattern), describing and measuring many their features and, finally, developing a typology of these reaches. This research was carried out in the Czarny Dunajec River channel twice: in 1977 and 1999. The analysis of types of channel reaches in 1977 displayed that the channel system of the Czarny Dunajec River is complex and formed by different fluvial processes. After 22 years great changes in channel structure and dynamics were found. First of all, the length of reaches downcut into the bedrock and the number of reaches formed by incision increased. Simultaneously, the length of braided channel reaches was reduced dramatically. Such transformation of the Czarny Dunajec River channel as well as other Carpathian channels is not due to natural factors because neither essential change in hydrologic regime of the Carpathian rivers nor intensification of tectonic movements took place during the last 100 years. The reasons for channel changes are due to indirect and direct human impact, mainly land use change in mountain catchments, river-bed gravel mining and river training works. Land use change in Polish Carpathians has appeared since the second half of the 20th century and consisted in progressive converting arable land into meadows. An increase of the catchment storage capacity and decrease of sediment transport by sheetwash resulted in reduction in channel load and channel downcutting. However, the greatest channel transformation was probably due to gravel mining and river training works. Intensive river-bed gravel mining has begun after 1945. The gravel is used especially for building purposes. Consequently, the natural channel bed armouring protecting channel beds from incision, is destroyed. Also destroyed are river training structures and bridge piers. Elimination of macroinvertebrates resulting from removal of their natural living places is an ecological consequence of gravel mining. Furthermore, through stability loss of the channel bottom and banks the potential effects of flood discharges increase. Great channel changes have taken place as the result of training works, what is demonstrated in cases of the Biały Dunajec River and Mszanka River channels. These channels have been regulated since the beginning of the 20th century, but the most intensive river training works were undertaken in the 1960s and 1970s. The historical and present-day cartographic sources, archive river training projects, hydrological data as well as data from field mapping were used to identify the channel response to different types of river training. The influence of longitudinal river training structures on channel changes was demonstrated for the Biały Dunajec River channel reach in Szaflary where groynes were constructed in 1971. As a result of that work, the sinuous, locally braided, and up to 350 m broad channel was altered to the single-thread channel. The width of this channel is 30 m. The length of the reach decreased by 18%. As a consequence of the channelisation, the channel gradient and discharge energy increased which resulted in rapid downcutting. This process was most intensive in the first 6 years after river training. The channel was downcut from 1.0 m to 2.0 m at that time. The channel-bed lowering resulted in increase of the bank height and transformation of floodplain into terrace. As a consequence of river incision, the groynes were gradually destructed, what started the process of lateral channel migration. During the 1977 flood the groynes were completely destroyed and the channel underwent transformation. Channel changes after erection of transversal structure were analysed for the case of the Mszanka River channel reach in Mszana Górna with debris dam built in 1961. The debris dam broke the continuity of fluvial system and divided the channel into two reaches formed by different processes: deposition dominated upstream of the structure, whereas downcutting was the main process downstream. The scale and rate of both processes were greatest within the short time after regulation and at short distance from the debris dam. Within the first 6 years, the aggradation just upstream of the structure reached 4.0 m and degradation downstream reached 1.5 m. The channel downstream of the debris dam is presently downcut to the bedrock and still undergoes incision. The floodplain was transformed into the terrace. The influence of drop hydraulic structures on channel changes was demonstrated for the case of the Mszanka River channel reach in Mszana Górna where the concrete structures were built in three stages between 1977 and 2003. The channel pattern was altered from braided to single-thread and sinuous and the channel was straightened and narrowed from about 100 m to 35 m. 26 concrete drop structures were constructed along the longitudinal profile of this reach and the banks were reinforced. The area of the active channel decreased 5 times and the area of the gravel river bars was reduced 119 times. Despite the channel shortening, straightening and narrowing, the process of downcutting was not effective due to channel gradient reduction by the structures. This kind of river training proved to be “unfitted” to different water stages and discharges within the year. During low water stages the channel is too broad. The lateral bars are formed narrowing the channel cross-section. During floods the river channel is to narrow and the river tends to enlarge its width and sinuosity eroding its bottom and banks. The river training structures are being destroyed. The described river training structures proved to be not only harmful to environment, but also ineffective in the long term. The great change of channel geometry was the reason of “maladjustment” of a new river training path to the hydrological regime of the river resulting in changes of the channel morphology and dynamics, decline of biological conditions and often in destroying engineering structures. Many failures seemed to result from wrong approach to the problem of river management. The engineers very often considered channel reach separately from the whole river channel. The role of that individual reach in the fluvial system was not studied despite the fact that it is known that particular river reaches are not independent but form a complex system. Alterations made to one river channel reach lead often to changes in others. Presently, according to the Water Framework Directive, modern designs of river training works are recommended, imitating natural morphology and dynamics of channel systems. A very good example of such engineering structures is the rapid hydraulic structure with increased roughness, which fulfils the part of traditional drop structures, but, contrary to them, enables fish migration, favours the oxidation of water and stays in harmony with landscape. Restoration of rifflepool sequences, applied for rehabilitation of river channels, is the other modern engineering practice. Position of individual riffle and pools along the longitudinal profile of a natural gravel river channel is stable, what should be used while planning the location of transversal training structures (they are advised to be erected in places of natural riffles). Preservation of woody debris in river channels (so far removed in order to increase channel capacity) is the other good river channel management practice, since woody debris has profitable influence on morphological, hydraulic and biotic conditions of river channels. For similar reasons, the maintenance or reconstruction of gravel river bars and different channel microforms (for instance oversize grains) is strongly recommended. The channels of the Carpathian rivers are in bad ecological state. The channels are unstable due to anthropological changes in bedload supply and discharge regime. In order to improve the state of the Carpathian fluvial systems, one should take into account many environmental, historic and economic conditions which are specific for each channel. Proper determination of the range of activities and expected results must be the consequence of combined investigations of experts of many disciplines.