AVULSION-INITIATED, LATE HOLOCENE SOCIOCULTURAL REORGANIZATION IN THE SOUTHEASTERN FRASER/NOOKSACK LOWLAND: AN HYPOTHESIS

During the Late Holocene an approximately 50-km arc of the Southeastern Fraser/Nooksack Lowland, an area in the eastern Gulf of Georgia region including the Nooksack and Sumas Rivers, has undergone major environmental changes. New geological and archaeological evidence suggests three major episodes of Lower Nooksack River development: (1) Early to Mid Holocene Nooksack River flow north through the Sumas Basin into the Fraser River, characterized by floodplain aggradation by a meandering stream and a major Nooksack/Fraser River confluence at the northern terminus of the Sumas Valley; (2) Late Holocene interbasinal avulsion diverting Nooksack River flow west into the Strait of Georgia via an abandoned glacial outwash channel, characterized by floodplain aggradation and delta progradation in the lower reaches of the Nooksack and development of the historic Sumas Basin; (3) Historic modification of Nooksack and Sumas drainages. Prior to modification, major changes in fluvial regime were the result of unstable geomorphic conditions, flooding, and possibly tectonics and lahars. Whatever the trigger, the Late Holocene avulsion significantly transformed Southeastern Lowland riverine and riparian wetland landscapes. Considering the central role aquatic resources and watershed geography played in conditioning Coast Salish sociocultural systems, we hypothesize that avulsion-induced basin reconfiguration initiated reorganization of the Lowland sociocultural landscape, including subsistence and settlement strategies, intergroup relations, and language distribution.