Renaturalization Drives Hydromorphological Recovery in Degraded Gravel-Bed Streams in Poland

The systematic regulation of Polish gravel-bed watercourses, notably intensified in the latter half of the 20th century, coupled with extensive gravel extraction, have become one of the main factors leading to severe channel incision and degradation of hydromorphological features. This paper investigates river renaturalization as a pivotal strategy to restore channel-riparian water connectivity in incised gravel-bed streams of Southern Poland. The river restoration projects were categorized into passive and active techniques. Passive methods, though less common, involve cost-effective methods like the restoration of erodible corridors, island-braided channel patterns, and woody debris presence, while active methods include mainly fish passes, check dam lowering, and artificial riffles. A total of 27 major activities carried out on rivers of Southern Poland were assessed, revealing a trend towards comprehensive renaturalization in collaboration with scientists, pro-environment organizations, and local authorities and communities. Despite the lack of long-term data for most projects, results demonstrated sustained improvements in hydromorphological features, including the shallowing and stabilization of deeply incised channels. Using a natural section of the Czarny Dunajec river, a brief case study was presented to explain the mechanism of spontaneous river renaturalization. It was also shown that a good restoration project should take into account the views of all river stakeholders, anticipate possible development trajectories of a freely migrating river, and assess the potential benefits for both nature and people. Increased deposition of macroplastics together with woody debris in naturally widened river sections, as well as the possible remobilization of pollutants previously trapped in bank sediments, presents an additional challenge for future projects.