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distinguished. The beginning of the first goes back into prehistorical time, and was finished at the beginning of the 18th c. This long period was characterized by a very smali direct human influence on the hydrosphere and a very slowly-increasing indirect influence through clearing of forests.

In the second period comprising about 150 years a considerable increase in direct human influence on the water network occurred. Many stretches of river channels were straightened, and their upper courses were elongated by artificial ditches. The whole area was cut by ditches, and most lakes were included into the runoff network. Although drainage operations were primitive, their influence on water conditions was not indifferent as they became common. At the same time further changes in land use occurred which effected a slow transformation in water circulation. Although the changes occurring in it affected the whole region, bringing distinct results, they did not effect the water relationships, and conseąuently living naturę significantly. The region preserved its 'Polesie’ character — largely marshiness. With its poor population, inaccessible communication, Iow agricultural level and lack of industrial objects, this region was economically underdeveloped and constituted an area of interst largely for researchers appreciating its great natural richness.

The third period started only in the 1960’s, but it has brought the biggest changes. Their main cause was the building of the drainage system — the Wieprz—Krzna canal. Another factor in the changes is developing recreation, and a third one — coal mining. The conseąuences of these phenomena are changes in the ąuantity and circulation of water and its ąuality, as well as landscape changes. Water from the Wieprz river conveyed by the canal to the Lakę Region area increases local reserves. Collected in retention reservoirs it serves in irrigation of meadows and supplies lakes as well as fish ponds. It is difficult to estimate to what extent its increased ąuantity effects the magnitude of runoff and evaporation. However, both circulation components have increased.

The drainage operations, accomplished on a large scalę, particularly in the western and central part of the Lakę Region, have affected the circulation dynamics. Runoff has increased, surface runoff has been accelerated, surface and ground retention have decreased, and the percentage of underground runoff in relation to the total has increased. Another conseąuence is lowering of the first groundwater table. The scalę of this lowering is differentiated, locally it exceeds 1.5 m, a big value in the Lakę Region conditions.

Water surface lowering in lakes is also recorded, particularly in the south—west region. This process is supposed to be associated with the coal mining in this area because neither a climatological nor drainage factor can play a role here; coal has not been exploited long to fully support this assumption. However, an evident effect of mining is ground subsidence over the exploited coal and the formation of wetlands in the appearing land depressions.

The conveying of Wieprz river waters to the Lakę Region area, including