The study explores the reasons behind the relocation of the construction site for Fort Abakan from the mouth of the Abakan River, as initially planned, to the right bank of the Yenisei River, between two mountains, Unyuk and Turan. The shift of sand ridges, damming these rivers and changing their beds, is examined, and the locations of the projected forts are described. Written sources suggest that the Abakan and Yenisei beds as related systems changed their positions simultaneously, likely between 1691 and 1697 and definitely no earlier than 700–400 BC. Modern hydrological data suggest that processes that occurred in the region in the Early Modern Age were essentially like those that occurred in the Early Iron Age. The earlier date of the Abakan bed’s change is evidenced by the destruction of the 1st millennium BC Tagar sites near Sartykov village on the Abakan. At present, the Yenisei makes an abrupt eastward turn in that place, following the general direction of rivers in the region. D.A. Klements’s idea that after leaving the Western Sayan canyon, the Yenisei had flowed westwards is rejected. The change of location for the prospective fort was caused by the evolution of riverine systems of Western Siberia, specifically by the shift in the Abakan bed.