The topic of flood protection, including polders, is usually raised on the occasion of every major flood event and after some time, when some provisional remedies are applied, the discussion fades. Polders as hydro-technical facilities, completing the task of taking over the flood wave peak, arouse a lot of conflicting emotions. On one hand it is believed that these are objects that can effectively complement flood protection system and can positively impact the environment. In contrast, high costs, low efficiency and many other reasons, cause that this is not a popular form of flood protection. The computational part of this paper focuses on non-existent polder in Stężyca with a total capacity of about 24 million m3. The model of the Vistula River section taking into account the existence of the polder was made using HEC-RAS. Calculations related to the rate of filling the reservoir with three different variants of unsteady flow and different lengths of the sill at polder inlet. Simulations have shown that the extremal case would require an inlet with a length of approx. 400 m to fully fill the polder during the flood. In the second part of the paper positive and negative aspects of polders’ construction were discussed.