Summary:

A forest is the form of natural storage reservoir and it by this can be perceived as mainstay of small water retention. The prognoses of unfavourable climatic changes (growth of air temperatures, diminishing of precipitations, frequency increase of extreme weather phenomena) causing a necessity of compensations these the unfavourable phenomena by a technical and un-technical undertakings to increase water storage in forests. The building of devices of small retention has reason both: natural (i.a. assure in forest ecosystems biodiversity) and economic aspects. Indicatory cost water storage 1 m3 of water in objects of small retention is ten times smaller than in great storage reservoirs. Permission to building of objects of small retention is required for damming height 1,0 m or for every investment situated on protected areas. More than once its very increased the costs of investment. Therefore building a small objects is pragmatic, e.g. a cascade systems reservoirs. The building of objects damming and slowing outflow from catchments is focused to protection of the valuable natural habitats and it can be use full in strategy of protection of areas - may be the essential element Nature 2000 (e.g. forest bogs ecosystems, moist or flood plain forest). In Poland at present two complex projects relating in forests small retention are realized (Enlarging retention possibilities as well as counteraction flood and drought in forest ecosystems on lowland regions, Counteraction of water erosion on mountain regions and maintenance mountain streams in good condition). In the both projects about 250 forest inspectorates takes part, and the costs of realization of undertaking be estimated on about 350 million PLN. The both projects have to be realized till 2013 year. The realization of planned tasks relating the small water retention runs with a different difficulties. In the national programmes of small water retention till 2015 year was realized about 25% of planned works only (average annually). The quantitative estimation of small water retention is more than once difficult. In the present work it was passed how to estimate quantity of small water retention as the result of using weirs in watercourse from in small forest catchments of the Krajeńskie Lake District. Flood plain forests are the richest and highly picturesque forest ecosystems. Unfortunately, river regulation and flood control contribute to their degradation. In Poland only 0.2% of total area is covered by flood plain forests. The Uroczysko Warta forest district constitutes one of the most important clusters of these sites. The construction of the Jeziorsko reservoir in the middle course of the Warta River has contributed to changes in the river regime (reduced flooding areas and decreased flooding frequency). It seems that the use of simple land improvement systems, i.e. gates, river bars and culverts with flap check valves, may result in the recreation of advantageous hydrological conditions. In such a case storage volume of oxbow lakes would be increased and ground water would be maintained at a higher level over longer periods of time. Planned building of numerous dams on water-courses and devices to dam up water on reservoirs are cheapest methods of enlarging of water supplies in catchments. Build of these technical devices no doubt will contribute also to protection very valuable marshland biotopes, fulfilling mostly functions protective and recreational.