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drying, a change in hydraulic gradients and a shift of ground-watersheds. The zonę of Nadrybie Lakę would be included into the Świnka river basin, which at present is drained by the upper Piwonia river. The Usciwierz Depression and lakes with big natural and recreational advantages would then be in the rangę of the Świnka river drainage (Wilgat et al. 1987; Janiec et al. 1988; Michalczyk 1985).

A considerable threat to the environment is the discharge of minę waters and sewage exceeding admissible standards into surface reservoirs.

Another threat to the hydrosphere comes from the penetration of precipitation waters through heaps of Carboniferous waste-rocks. These waste-rocks are also distributed in the region to fili depressions without run-off and peat excavations. In the neighbourhood of Piaseczno Lakę the specific conductivity of waters percolating these waste-rocks is ten times higher than that of lakę waters.

AN ATTEMPT AT DETERMINING CHANGES IN THE OCCURRENCE AND CIRCULATION OF WATER

Hydrographic changes can be determined by various methods:

a) One of them is comparing maps issued in different periods. Thus — determined changes to the drainage network were presented in chapter Hydrological works (p. 125). Maps allow us to find most lakes included into the drainage network. It is much morę difficult to determine differences in the surface area of lakes, because the shore contours are not distinct due to a vegetation belt occurring around them. However, the decline of some of the lakes can be found (besides the described Lejno Lakę, Blizny Lakę, which was shown on map at the end of the 19th c., has already disappeared), as well as the shrinking of the surface area of others. There are two reasons for the latter: water surface lowering after digging runoff ditches and plant overgrowth of lakes. Field studies, however, give morę data about disappearence of lakes than the comparing of maps.

Maps show a big increase of runoff network density. No attempt has been madę to present this problem in numbers because of the inadeąuate accuracy of maps showing ditch courses. Nevertheless, it can be seen that all areas of the Iow accumulation plain have been cut by ditches connected with the river network. The building of the Wieprz—Krzna canal with all its waterducts caused the biggest change in the water system of this region.

The rangę of wetlands is an important problem in the evaluation of hydrographical changes. An analysis of the land character and its utilization was madę for three time periods in the central part of the Lakę Region, which was mentioned in chapter Changes in landuse... (p. 122). Its results are presented in Table 1 and Fig. 3. Despite some reservations as to the exactness of distinguishing various kinds of land on maps, the changes can