Geomorphological effects of beaver activities in lowland drainage ditches

Drainage ditches arc 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: ( I) 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 m3. 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.