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The influence of anthropopressure..

into the Oder the free surface of water was only lowered within a span of 110 km, while it was raised along a 150 km section and remained unchanged along a 148 km section (Lambor 1951).

Attempts to achieve maximal increase in the size of the areas under cultivation through deforestation or through dewatering of marshy areas or open water reservoirs had an adverse effect on the water cycle. Similarly, regulation of rivers consisting mainly in the straightening of the naturally meandering river beds resulted in an increase of elevational drop and of the speed of water runoff. It caused deepening of river beds and, in conseąuence, lowering of the level of underground waters. Soon, often as early as m the next generation’s lifetime, newly-reclaimed cultivated land turned out to be excessively dry. For instance, as a result of the land melioration in Kujawy the underground water level fell by several meters. In wide, previously marshy, river valleys the underground water level in summer months often falls below 2 m (Kaniecki 1991).

Low stages of watercourses are a reflection of the underground water supply. Z. Pasławski states (1964) that according to the water level indicator in Skwierzyna on the Warta river in the years 1870—1962 the average level of low stages fell by 18 cm. At the same time the average duration of low stages increased from 75 days at the beginning of the studied period to 192 days at its end, that is almost three times. Minimal water levels were shifted from July to September. Moreover, some of the minor rivers disappeared. These were: the Goplenica, starting at Gopło Lakę and mentioned by Jan

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Długosz, the Elsse near Świebodzin, the Oder’s tributary and the Zielona Struga starting in the peat-bogs in the Dziemionna valley in the vicinity of Inowrocław (Falkowski, Karłowska 1961).

A part.cularly intensive process of reduction of the river network is evident in urban areas. At the turn of the 19th and 20th centuries the following watercourses ceased to exist: the Zgniła Warta, Struga Karmelicka and Struga Rybacka as well as the lower courses of the Wierzbak and Bogdanka rivers (Kaniecki 1993). This tendency is still evident — in the 20th century the following watercourses disappeared: the Bystry Rów, Obrzyca, Piaśnica and its tributaries: the Pokrzywka, Zegrzynka and Chartynia, directed to the underground sewage system (Kowalik 1995). In the rural areas, for instance in the vicinity of Poznań or Gniezno, "piping" of minor watercourses, making agricultural work difficult, is practised. The majority of the rivers (as evidenced on the bas s of hydrographic charting carried out currently in the Wielkopolska Lowland).run waters periodically. In the summer months even rivers draining areas of s?veral hundred sąuare kilometres run water only from the middle of their ccurses.

Intens.ve dewatering of both uplands and valleys resulted in lowering of the wa:er level in lakes and freąuently in their complete disappearance. The survey of the 18th century maps drawn by Zanoni, Gilly and Fan, that is from tha period preceding intensive land melioration, proves that around 30 lakes lisappeared completely from the Wielkopolska Lowland. At the