Changes in Water Resources in Selected Lakes in the Middle and Lower Catchment of the River Warta

Lakes play a very important function in water retention, particularly in areas with strong water deficiency. Water resources in lakes determine the possibility of using them for different purposes, e.g., economic development (irrigation or drinking water, water supply for industry). With ongoing processes causing a reduction in lake capacity (shrinking and shallowing), it is necessary to investigate the scale and rate of decreasing lake resources, in terms of the economic and natural consequences of this process. The aim of the study was to test changes in water resources in lakes situated in the middle and lower River Warta catchment. Special attention was paid to lakes characterised by reduced water resources in order to identify factors contributing to this phenomenon. Our current studies of 127 lakes showed a 10% decrease in water resources over 50 years (1910–1960) in the analysed part of the catchment. Our results indicated that the predominant process causing the disappearance of lakes is shallowing. This factor should be monitored carefully to avoid further lake loss.