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The influence of anthropopressure..

At the close of the 16th century, the so-called "Dutch colonisation" ("kolonizacja holenderska"), aiming at bringing wetlands under cultivation, started in the Wielkopolska Lowland. The area of plough-land was increased by the drainage of the excess water from the boggy or partly-flooded areas. Dutchmen indeed were the first colonists who brought under cultivation vast areas of wetlands, often overgrown with black alder bog forests and shrubs. They were eąuipped with many centuries of experience in that field.

Later on the "Dutch colonisation", which in the 15th century started in the

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region of Żuławy Gdańskie, and subseąuently spread over Prusy Królewskie and Prusy Książęce and progressed towards the south along the Vistula valley, was continued mainly by German immigrants. In the region of the present Ciechocinek on the Vistula, "Dutch" settlements extended in a wide belt across the Kujawy, especially in the vicinity of larger lakes, and entered the Warta river valley where their number reached several tens between Kolo and Oborniki Wielkopolskie (Baranowski 1915). The oldest preserved location privilege from the historical region of Poznań province refers to Olędry Ujskie. The village was established in the year 1597. Other settlements of this type were built in the areas adjacent to the Noteć and Warta estuaries (Rusiński 1947).

The "Dutch colonisation" developed on a particularly large scalę at the close of the 17th century and lasted up to the partitions of Poland. For instance, in the Kościan administrative district, all "Dutch settlements" are dated to the 18th century The highest concentrations of settlements of this type came in to existence in the Noteć river valley between Czarnków and Wieleń, on the Warta river in the vicinity of Międzychód and to the east of the Obra river, on the Warta between Śrem and Pyzdry, and in a wide belt extending along the Czerniejewo-Murowana Goślina-Rogoźno-Chodzież linę. All these settlements were located in the former Kościan and Poznań administrative districts, while nonę of them were established in the Walcz administrative district and in the district of Wschowa. In 1789, the number of unąuestionably "Dutch" settlements amounted to 156 in Poznań administrative district and to 111 in Kościan administrative district (Rusiński 1947). Apart from being located in the valleys of big rivers and on shores of all types of inland waters, "Dutch" settlements were sometimes built on fiat, outwash wetlands, for instance near Nowy Tomyśl, and in higher, dry areas where the land for farming was gained by way of tree logging. However, the building of settlements in wet areas and their dewatering prevailed.

Within the wet areas water was directed to rivers or lakes by means of ditches or was accumulated in deep ditches and pits. In spite of the high intensity of "Dutch" colonisation, the process of the dewatering of wetlands was relatively slow and covered relatively smali areas. Vast areas of marshes remained untouched.

River engineering entailing in the straightening of the courses of rivers, moving river-beds, clearing and protection of their banks, initially for defence reasons, and then impounding of their waters enabling the work of plants