We report new interpretation of >19,500 beach strandlines from waterbodies in the western St. Lawrence and Champlain Lowlands in northern New York and adjacent areas of Vermont, Quebec, and Ontario from ≤2-m-resolution digital elevation models. Strandline evidence supports a deglaciation model in which proglacial lake and marine shoreline deposits adjusted continuously in response to steady shoreline regression linked to outlet incision, differential isostatic adjustments, and postglacial relative sea-level rise. Gaps in strandline preservation reflect times of rapid water-level decline associated with breakout floods and abrupt shifts in drainage to new outlets. Water levels returned to slower, steady decline and renewed beach sedimentation during the later stages of a breakout as water levels in the source and receiving waterbodies equilibrated. Our conclusions contrast with previous models that infer discrete lake stages were controlled by stable outlets then fell abruptly as lower outlets were exhumed from beneath the Laurentide Ice Sheet during deglaciation. We present a new deglacial chronology and lake nomenclature that reflects this paradigm and redefines the spatial and temporal distributions of proglacial lake and marine water in the region.