with 8 figures and 6 tables Summary. Drainage ditches are the result of a large-scale anthropogenic transformation of the hydrographical network in the agrarian landscapes of European lowlands. These drainage ditches are readily occupied and transformed by beavers for their own needs, due to the paucity of remaining intact natural watercourses. The geomorphic consequences of beaver activities in inhabited streams are quite well documented in mountain environments of North America, especially for various aspects of sedimentation. There is, however, a lack of knowledge concerning other geomorphic effects resulting from beaver activities in the drainage ditches within agrarian lowlands. To assess these geomorphic effects, an investigation was conducted in the anthropogenically transformed landscape of the Middle Lithuanian Lowland to: (1) quantify the geomorphic effects, and (2) predict the shifts in the magnitude of total geomorphic effect as the size of beaver population changes. Two methods, aerial photographs and global positioning system, were used to map all ditches in selected sites and all available beaver-created visible structures therein. On the drainage ditch network scale, the quantitative geomorphic effect of beaver activities resulting from construction of dams, burrowing into the ditch slopes, and digging of lateral canals, as well as the development of scours where the beaver dams abut the drainage ditch slopes, was estimated to average about 35 m 3 km-1 of relocated substances. However, on the local scale, within one beaver-site, the total geomorphic effect was much greater, up to about 176 m 3. On the catchment scale of the whole hydrographical network, an increase in either the length of dammed drainage ditches by 10%, or the density of beaver-sites by 0.1 ps.km-2 , resulted in the potential growth of the total geomorphic effect by about 15 and 11 m 3 km-2 respectively. These quantities should be enhanced by about 6.5 times if the estimate of total geomorphic effect includes sedimentation in beaver ponds.

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Most natural streams were made into drains when they were converted into recipients of subsurface tile systems. During the process the variety of biotic and abiotic conditions within and around the streams was totally destroyed, and the new watercourses (open drain recipients) were deprived of the greater part of the ecological importance that the former streams had possessed in the landscape. The data collected over the last decade in Lithuania about drain bed deformations, sediment accumulation, vegetation cover successions, intentional afforestation of slopes, beaver expansion and succeeding water quality changes, revealed some self-perpetuating natural alterations which raise the ecological significance of open drains as refuges and corridors in the landscape for vegetation (the succession of herbage and appearance of woody vegetation on slopes), and the barriers protecting surface runoff from diffuse agricultural pollution (beaver impoundments). On the other hand, these alterations are useful in terms of maintenance of the functioning of those drains as recipients because they result in steadier (roughly parabolashaped) cross-section profiles, less threat of bed silting up and no need for the mowing of slopes (because of the growth of woody vegetation). This paper presents and discusses the study results highlighting the above-mentioned bidirectional effect of self-renaturalization, and considers the preconditions RÉ SUMÉ La plupart des écoulements naturels ont été canalisés dans des drains lors de leur conversion en collecteurs des flux de drainage souterrain. A cette occasion, la variété des conditions biotiques et abiotiques internes et externes aux écoulements a été totalement détruite, et les nouvelles voies d'eau (collecteurs à ciel ouvert) ont été privées de l'essentiel du contenu écologique des ruissellements naturels. Les données rassemblées depuis dix ans en Lithuanie sur les déformations des collecteurs, l'accumulation des sédiments, les successions du couvert végétal, le reboisement intentionnel des pentes, la prolifération des castors et les changements de qualité de l'eau, ont révélé quelques unes des altérations autoentretenues qui posent la question du poids écologique des drains à ciel ouvert comme refuge et couloirs de végétaux dans le paysage (succession des herbes et apparence de végétation boisée sur les pentes) et barrières protégeant le ruissellement de surface de la pollution agricole diffuse (barrages de castors). D'un autre côté, ces altérations sont utiles en termes de maintenance des drains collecteurs parce qu'elles conduisent à des profils en travers plus stables (forme grossière de parabole), une moindre menace pour l'envasement du fonds et un moindre besoin de tondre les pentes (présence de végétation boisée). Cet article présente et discute les résultats d'étude qui mettent en évidence les impacts bidirectionnels de la re-naturalisation

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In the process of high expansion of drainage systems and land cultivation, most natural watercourses in Lithuania were converted into drainage channels. They were deprived of all natural riparian vegetation and abiotic conditions that kept up biodiversity, diminished erosion and deflation, induced self-purification of water etc. Thus the re-naturalization of drainage channels is essential. Consequently, to determine the possibilities to combine re-naturalization processes in channels with their function-ability it was examined the tendencies of woody vegetation (WV) spreading and the hydraulic calculation formulas estimating the performance of overgrown channels as water recipients were suggested. As the study results showed, self-restoration of WV was most progressive in forest and outskirt channels. In the field channels WV expansion takes place as well. However, it depends on the distance from a forest, i.e. WV density decreases sharply from 1.17 stems m−2 to background level (c. 0.26 stems m−2) within the space of 300 m. In the paper there is some positive influence of WV cover in view to drainage channel maintenance discussed referring to the results of former studies as well as the publications of other authors. It includes the prevention of bottom silting as well as slope erosion and slide. The formulas estimating conductivity of overgrown channels are based on the balance of forces occurring on the boundaries of overgrown strips of a stream: resistance of tree stems, friction on the slopes, shear stresses that appear when two streams with different velocities are in contact and the horizontal component of the gravity force causing water movement. Some methods of WV cover abundance control are discussed when having assumed self- or artificial restoration of WV in channels to be maintained as recipients is projected.