This study investigates environmental change as a context for the location of occupation in campsites along the lower Nottawasaga River in southern Ontario, Canada. Evidence of Archaic and Woodland period campsites presents a difficult scenario for archaeological interpretation. Did Archaic cultures prefer to occupy higher ground, such as on the Edenvale moraine (206 m above sea level [a.s.l.]), or lower areas within river valleys, such as on the floodplain (181 m a.s.l.)? The aim of this study is to disentangle the evidence from a geoarchaeological perspective, considering cultural sites cross-temporally in their physical setting of a river floodplain in the Great Lakes region. We used geoarchaeological methods and methodologies, including soil-sediment analysis, sedimentological profiling of cutbanks and cores, topographic profiling in the field, river morphometric and hydrometric measurements using various techniques, ground-penetrating radar (GPR), and conceptual modeling. Results indicate that the Edenvale moraine, which topographically confines the channel and floodplain in the study area, may have led to catastrophic stripping of the floodplain during drainage of glacial lakes, causing the Nottawasaga River to downcut into the Edenvale moraine. This occurred following a period of floodplain stability, when natural and cultural deposits would have been preserved and buried in confined sections of the floodplain that are currently being vertically accreted. Any model that considers accretion style, such as the preservation-exposure model presented here, should be interpreted within the context of the environmental history of the area, since accretion style varies temporally and spatially at the river-reach scale.