Abstract The investigation of oxbow lakes conducted in the middle section of the River Łyna valley, which is representative of the post-glacial landscape in northeastern Poland, showed a susceptibility to degradation with regards to hydrological connectivity and external sources of supply. When the connection to a river is maintained, oxbow lakes prolong their existence, thanks to multiple exchange of matter. When oxbow lakes are disconnected, their degradation is much faster and they undergo advanced hypertrophy, overgrowth and eventually disappearance. The process is accelerated when an oxbow lake becomes a trap for nutrient effluents from reclamation systems that dewater highly-managed arable lands or grasslands. In agricultural river valley landscapes, oxbow lakes may function as very effective biogeochemical filters, contributing to sediment trapping, nutrient removal and river pollution prevention. However, the benefits are unilateral: ecological equilibrium in the oxbow lake ecosystem is easily lost.