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A. Kaniecki

the cleaning-out of a river bed to enable the work of water mills, could have the contrary effect. The inhabitants of the Wielkopolska Lowland built embankments and exerted an influence on the shape of rivers as far back as in the early Middle Ages.

Due to the lack of a chronological compilation of archaeological findings and a lack of studies on the history of settlement in the Wielkopolska Lowland, a detailed reconstruction of changes in water relations in earlier centuries is still impossible. Even the results of the research on the changes in water level in Goplo Lakę in the early Middle Ages do not allow for generalisations, sińce they could be caused by both natural factors of local character as well as by human activity. One should also take into account local effects of neotectonic movements, for example uplifting movements in the Kruszwica region resulting from the tectonic structure of Goplo Lakę, or downward movements in the region of Poznań. Although in the scalę of one year the intensity of these movements is smali, in a millennium they could bring about changes influencing the hydrographic system and humidity relations. So far we have not discovered the mechanism of such transformations of water relations. In conseąuence we are not able to explain why the settlements located by lakes, dated back to the early Middle Ages are placed in some areas high above the present water level and in others below that level.

Up to the lOth century, man’s economic activity had a relatively smali influence on environmental changes and covered as little as 10—20% of the Wielkopolska Lowland, with only 5-6% of the area affected by intensive exploitation (Kurnatowski 1975). At that time, settlement and agricultural activity concentrated mainly in river valleys and in lakę gullies. Early settlers were attracted by the ease of farming on alluvial soils, formed predominantly on the layers of sand covering the bottoms of river valleys which were additionally fertilised by annual flood waters. At the end of the last millennium the level of underground waters in river valleys was at least 1 metre below the present level, a situation which can be inferred from the fact that most settlements were then located at the upper levels of the flood terrace. Moreover, archaeological research conducted in the Obra river valley revealed that lower parts of dug-outs and fortifications were situated 1 to 2 metres below the present level of groundwaters (Kurnatowski 1968).

Due to the increasing humidification of climate entailing a rise of underground as well as surface waters, the process of settlement moved from valleys to upland regions which were adapted for agricultural purposes, mainly through deforestation. After the 13th century the process was facilitated by the introduction of improved agricultural tools by German settlers, which helped put heavier soils under cultivation. The decrease in the area overgrown with forests was often accompanied by the self-renewal of forests on the land abandoned by farmers. However, the new forests were considerably poorer in species. People still did not know how to adapt wetlands for farming. At the end of the 13th century, river valleys were used mainly