Aim of the study The paper presents an assessment of capacity of a drop hydraulic structure, which was partially destroyed, and provides a description of slow renaturalization of a riverbed in this area. As a result of damage to the hydrotechnical structure caused by natural processes, some elements detached from the structure were deposited in an energy dissipation basin and in the riverbed downstream of the structure, impeding rock and vegetal debris traffic. These fluvial processes formed a cascade-like structure that led to a change in the regime of water flow through the hydrotechnical structure that was not planned or designed originally. Material and methods Due to partial destruction of the structure, the water flow regime was so drastically altered that it is possible for various species of fish to cross this section of river without any additional constructions like fish passes. Field measurements determined six potential fish migration routes through the damaged structure’s area. Then each fish migration route was evaluated against the possibility for fish to pass through the structure. Results and conclusions The analysis showed that the partly damaged drop hydraulic structure creates favourable conditions for migration of some fish species. It can be concluded that the elements detached from the drop hydraulic structure and deposited downstream should not be removed from the riverbed, because they improve the environmental conditions for aquatic organisms in terms of their migration.
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