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Many of Poland's valleys resemble those of the Mała Panew and Liswarta (and their tributaries) in featuring the remains of old hydrotechnical infrastructure formerly used by gristmills, steel plants and sawmills. As the importance of water as their main source of energy declined, such earlier industrial plants were gradually closed down, with a steady process of deterioration of hydrotechnical infrastructure then set in train. Beavers were reintroduced into the catchments of the Mała Panew and Liswarta in the 1990s, with the rodents immediately commencing with the adaptation of anthropogenically-transformed habitats to better meet their needs. Against this background, research was conducted to determine:. • the number of beaver sites in the river valleys in question, and especially the number within or near disused items of hydrotechnical infrastructure,. • the ways in which beavers have "managed" the infrastructural items in question,. • the impact beavers' activities have had on the environment in the valleys studied. The locations and functions of old items of hydrotechnical infrastructure were determined by reference to archival maps. To assess beavers' activities, 6 sections of watercourse of lengths 0.5-7 km were selected, with locations of beaver dams and ponds, as well as other traces of activity by the animals, documented and described. With a view to findings being summarized, distinctions were drawn between various methods of adaptation of former hydrotechnical objects by beavers, i.e. 1 - dams built in the channels of regulated rivers, 2 - dams built in culverts, 3 - former millrace/water channels (used by gristmills or ironworks) occupied and adapted, 4 - banks of retention ponds occupied, 5 - waters in river channels dammed and basins of old ponds flooded, 6 - banks of ponds serving fire-fighting purposes occupied, 7 - former pond dams restored. Beavers can adapt successfully to - and also themselves adapt - anthropogenicallytransformed environments in river valleys. In this regard, the rodents were found to be more eager to occupy the tributaries of the Mała Panew and Liswarta than the rivers themselves. The valleys offer suitable conditions for beavers, given channels that are shallow (of 0.3-0.6 m), slightly sloping (2.2-6.7 m/km), with sandy or sandy-sludgy bottoms, as well as access to deciduous trees growing nearby. Along the Mała Panew and Liswarta themselves, beavers make use of side channels and millraces. Most numerous traces of beavers' activity were to be observed along forested stretches of the Leńca and Olszynka Valleys, which are found to have been transformed by beavers along 28-35% of their lengths. The animals made instinctive use of relic items of hydrotechnical infrastructure, with an interesting case involving the "restoration" by beavers of old embankments of anthropogenic ponds on the Rów Kokocki and the Leńca. Above beaver ponds (on the Rów Kokocki) or between ponds (on the Olszynka and Leńca), reduced channel slope and increased channel sinuosity contribute to the creation of side channels. Along these sections, transport channels made by beavers are vulnerable to avulsion and the emergence of multi-channel sections of watercourses. [ABSTRACT FROM AUTHOR]