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A. Kaniecki

driven by water wheels, also affected water relations in the adjacent areas and the intensity of river-bed processes (Fig. 2). The processes were particul-arly clearly evident in the areas adjacent to larger towns, where water mills were most freąuent (Fig. 3). The adaptation of watercourses for the needs of water mills involved first of all dredging of their beds and damming-up of their waters. As far as the change of water relations is concerned the conseąuences of these activities varied. Damming-up of watercourse waters resulted in the rise of the underground water level in adjacent areas, while the dredging of river-beds and the digging of drainage ditches in wetlands were responsible for lowering of the water table.

Within lowlands, where only a slight elevational drop occurred and where in summer, due to Iow flow, water- courses freąuently lost their waters in boggy segments of valleys, their resources were increased by supplying water from lakes, other watercourses or bogs by means of canals and ditches. In the course of time changes of this type, initially taking place in areas adjacent to towns, spread over increasingly large

Fig. 2. Changes in the relief and watercourse

network in the vicinity of Poznań A— around the middle of the 13th century,

B — in the 14th century, C — in the 18th century, G — Grobla; C — Czartoria, OT — Ostrów

f

Tumski, S — Spytkowo, R — Środka, 1 — boul-der clays of the upper terrace level, 2 — alluvial sands on river islands (Kaniecki 1993), 3 — the

boundary of cities