Numerous sites from the Roman and Early Medieval period have been discovered on the flood terraces of river valleys in central Poland. Settlement in these areas would have been unexplainable under present climatic conditions in view of the high ground water level and flood risk. Studies of the natural environment have accompanied archaeological work carried out in the Prosna valley, on an Early Medieval site in Kalisz (strongholds in the districts of Zawodzie and Ogrody, as well as a site in the Old Town), and in the valley of the Mogielanka), on a cult site from the period of Roman influence at Otalazka near Mogielnica. In both cases, the features are located on wide, flat and low flood terraces, which are now either artificially drained or else remain unused as boggy meadows. On the ground of comparative studies carried out by paleobotanists, limnologists and meteorologists among others, the conclusion can be drawn that conditions suitable for settlement in river valleys existed in Poland during the climatic optima, that is, in the Roman and Early Medieval times. These periods were separated by phases of unfavorable climatic change at the end of the 5th and early 6th century and in the 6th century. Violent floods led to the destruction of sites from the period of Roman influence located on the low terraces near river beds. In this cool and humid period, the population living in the low terraces of rivers in central Poland was forced to move, the action occasionally turning even into mass migration. The outcome was a population drop. The return of settlement to the low terraces occurred once climatic conditions improved to some extent in the 7th-8th centuries; this situation lasted until the 12th century. In the 13th-14th centuries the climate deteriorated again (small glacial age), resulting in an exodus of the population to the terraces above the floodplain and the uplands, where chartered towns began to be established. 7 Figures.