Galveston Bay formed above the Trinity incised valley as it submerged in response to Holocene sea-level rise. Seismic, sedimentologic, and radiocarbon data indicate that Galveston Bay did not form at a continuous rate during the Holocene. Rather, its evolution is punctuated by periods of rapid environmental change. These periods of rapid change are preserved in the Trinity incised valley as flooding surfaces. A nearly flat, prominent flooding surface has been recognized and mapped regionally at about -10 m within the bay. The flooding surface was sampled at a variety of locations along the axis of the Trinity incised valley by rotary drill cores collected from a shallow draft barge. Maps of the paleoenvironments bound by the flooding surface and radiocarbon age dates indicate that the environmental retreat event resulted in estuarine environments being shifted about 25 km landward in less than 1000 years. The interaction between Holocene sea-level rise and fluvial paleogeomorphology has been interpreted as the forcing mechanism behind the environmental change event. At approximately 7,200 cal. BP, sea level reached the top of a fluvial terrace in Galveston Bay. This terrace acted as a threshold to coastal retreat, and once sea level inundated the flat terrace top, rates of bay-line retreat increased dramatically. This event impacted the entire bay complex from the barrier shoreline to the bay-head delta and is a good example of how relatively small (decimeter) changes in sea level can have a large impact on coastal systems.