Geomorphologic and sediment-stratigraphic study of the Koster site has been carried out in the broader context of the lower Illinois Valley. Accumulation of reworked loess in an overdeepened tributary valley began at Koster shortly after 10,000 B.P., and continued through Holocene times with major sedimentary breaks. The Illinois floodplain began to stabilize ca. 5000 B.P. after rapid aggradation, but remained a dynamic environment that developed its present patterns after 2500 B.P. Valley-margin hillside vegetation was considerably more xeric during the periods 1200-950 B.P., 2100–1900 B.P., and ca. 9700–5000 B.P., with hillside woodland reduced to hill prairie or parkland ca. 8500–7700 B.P. These dramatic Holocene environmental changes suggest that interpretative archaeological models for cultural adaptations through time must consider the environment as a critical variable, rather than as a constant.