Working accordingly to their tasks hydrodynamic structures tends to negatively modify the river bed disturbing its function as ecological corridor for migration of living organisms between separated areas laying up and down the river. To neutralize to the certain point the negative influence the hydro-technical structures has to be supported with devices helping the living organisms in the migration. The main problem on this structures is to handle the great difference between the water levels before and after the structure and with it a lot of potential energy. While accomplishing this main task as less as possible from available free space and discharge can be taken. The costs needs to be limited and the proper conditions for migration of living organisms have to be secured. Fish ladders are used for this purpose. Classical solutions in this constructions have proven to be unsuccessful and the biological fish ladders require a lot of space as well as unacceptable portion of discharge and are cost-ineffective. The optimal solution balancing usefulness and limiting the negative effects can be achieved be using the rock fishways.

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