Dam reservoirs constitute an important element of protection against floods and hydrological droughts, and they ensure the possibility of producing electricity. Loss of reservoirs’ storage capacity has a significant impact on the management of their water resources, including flood protection and counteracting the effects of drought and the possibility of producing electricity. The paper presents changes in the capacity of 47 reservoirs in Poland that have the status of key objects of protection against floods and hydrological drought. Based on the collected, unpublished data, the changes in capacity from the beginning of the reservoirs’ existence to 31 March 2021 were calculated, which allowed us to determine the total amount of lost capacity and the pace of the processes taking place. From the beginning of operation (average operation time 48 years), the capacity has decreased by about 5%, which means that almost 200 million m³ less water is stored. Detailed analyses of the lost capacity also allowed for an illustrative presentation of forecasts for further changes in the short and long term. The results obtained represent a unique contribution to future national strategies for the management of sediment and reservoirs’ flood reserve and reduction of drought. The presentation of this problem seems to be important also in the context of climate change.

The proper characteristics of the catchment area of dammed reservoirs is of great importance when managing their capacity in the context of enabling proper functioning, including retention of the planned amount of water and counteracting the effects of drought. Therefore, detailed analyses covered mountain reservoirs with varying degrees of capacity changes: Goczałkowice on the Vistula, Rożnów on the Dunajec and Tresna on the Soła, thanks to which reasons for the differences in these changes were determined. For this purpose, data on the volume of the suspended load was used. Diversity in its transport is mainly caused by the presence of damming structures. Correlation of morphological and hydraulic indicators was carried out, preceded by the identification and characteristics of all transverse and longitudinal structures located in catchments upstream of individual reservoirs. Analysis of the geological structure, as well as the structure of land use in the basins of the reservoirs, was also performed. The obtained results allowed for the preparation of recommendations for further work, allowing, among other things, an increase in water retention. The key factors for maintaining the capacity of retention reservoirs were defined, which may be useful in national plans or programs in the field of counteracting the effects of drought or flood protection.

Maintaining the appropriate capacity of a retention reservoir is necessary for the optimal performance of the functions for which it was built, including flood and drought protection. Therefore, to properly manage individual reservoirs and sediments within their catchments, it is necessary to analyze the factors affecting the reduction of the retention capacity of reservoirs. Our study proposes a methodology for conducting a multi-criteria assessment of anthropogenic pressures and natural impacts affecting the reduction of reservoir capacity, such as land use, hydrographic network density, hydraulic development, and land slopes. For this purpose, geospatial analyses were applied to a grid of basic fields (hexagons). The research procedure showed that land slopes in the catchment area are the key factor determining the supply of sediment to the reservoir. Our study focused on the basins of reservoirs located in the southern part of Poland: Goczałkowice on the Vistula, Rożnów on the Dunajec, and Tresna on the Soła. However, our proposed new approach to multi-criteria assessment of reservoirs can be applied to and implemented in other catchments. The application of solutions based on our study may contribute to maintaining or potentially increasing the level of water retention in reservoirs and their catchment areas.

Multifunctional flood protection reservoirs of regional importance have been controversial for many years due to their significant environmental impact. The impact is especially observed at the implementation stage. Undeniably, properly managed large water reservoirs are the most effective element of active flood protection and significantly reduce the flood wave, thus limiting losses in housing, infrastructure, industry, or agriculture. At the same time, multifunctional reservoirs pursue other goals that are related to reducing shortages and counteracting the effects of drought, water supply, hydropower, or recreation. In the era of rapidly advancing climate and geopolitical changes, their role seems indispensable when it comes to responsible and coordinated water management in Poland. An open question is how to fully use the potential and to what extent it is possible for the system of reservoirs located on different watercourses to work together in the era of the progressive development of the country resulting in significant changes in land use (sealing large catchment areas) in relation to design assumptions. The article presents an analysis of the work of selected water reservoirs located in the basin of the upper Vistula during the floods that took place in the last decade. On the basis of the obtained conclusions, the authors present recommendations on how to effectively manage the network of reservoirs during a flood while minimizing losses of the water retained in them, which should be used to meet other goals of water management.

Retention reservoirs are one of the basic elements of water management. Multifunctional, they are used for flood protection, drought prevention and other purposes. Water reservoirs can also play a fundamental role in improving navigability on the lower section of the Vistula River from the Włocławek barrage to the mouth of the Gdansk Bay. Their implementation is a determinant for the possibility of restoring the economic role of the river in question. Therefore, the purpose of this research was to determine the current navigation conditions and the state of the regulatory buildings on the Lower Vistula. The investments necessary to achieve international parameters of the waterway were also determined, together with the calculation of financial benefits related to their implementation. This was possible based on the conducted bathymetric measurements, spatial analyses, economic calculations and queries of the obtained studies. It was established that fairway depths of 1.4 m are ensured for 180 days per year on average. On the other hand, the construction of five barrages (Siarzewo, Solec Kujawski, Chełmno, Grudziądz and Gniew) will make it possible to achieve at least a navigability class IV (international) on the section of the Vistula River waterway. In addition, the barrages will enable the retention of 1.2 billion m³ of water and will be associated with financial benefits such as shipping use, flood protection and counteracting the effects of drought. On this account, annual revenues may amount to approximately PLN 3.5 billion. This shows that the construction of the reservoirs is profitable, and the proposed solutions should be implemented in the context of sustainable water management and adaptation to climate change.

The following databases contains information on land use with water in Poland: Corine Land Cover (CLC), the Urban Atlas (UA); Database of Topographic Objects (BDOT) the digital Map of Poland’s Hydrographic Division (MPHP); and the Register of Lands and Buildings (EGiB). All these data are referenced in scientific analyses and the Polish water management system, so the results of their processing should be the same (or at least similar); if not, output materials will be inconsistent and unreliable. In the Katowice sample, we checked the quality of this data using multi-criteria analyses, which is based on a grid of equal-area hexagons. Additionally, we applied the Normalized Difference Water Index to check real-time water presence. We detected discrepancies between all the data. The CLC does not reference any flowing water in Katowice. Most data overlapped between MPHP and BDOT, and both databases were similar to UA. However, a lot of uncertainty was also observed in the EGiB, which is considered to be the most accurate of the databases surveyed. In conclusion, we argue that water land cover data should be used with caution, and depending on the scales of analysis, that most actual data could be remote sensed data. We also include a diagram which can be useful in the data selection process.