EVIDENCE OF A MAJOR LATE HOLOCENE RIVER AVULSION

The late Holocene avulsion of a major river from one basin to another has great implication for international interest because of concerns with cross-border flooding, volcanic hazards and future avulsion potential, but has had little study to date. The Sumas Valley, located in the eastern portion of the Fraser Lowland, encompasses portions of Washington State and British Columbia. The valley has a complex Quaternary geologic history consisting of continental glaciation, structural activity, and more recent alluvial deposition by three major river systems. Earlier work demonstrated that thick deposits of bedload originating from the Mount Baker volcano underlie the Sumas River valley, which suggests that the Nooksack River, which heads on Mount Baker, flowed north through the Sumas Valley into the Fraser River for much of the Holocene. The Nooksack River drains more than 1400 square kilometers at the point where it would flow north to the Fraser, making it one of the larger tributaries to the Fraser. At some point in the Late Holocene the Nooksack River avulsed from the Sumas Valley into a remnant glacial outwash channel that is its present channel course into Bellingham Bay, some 58 kilometers south of where the Fraser meets the Pacific Ocean. Numerous relict channels and oxbows in the Sumas valley, consistent in size and radius of curvature with the modern Nooksack River, the depth and distribution of flood deposits, as well as alluvial fans that have been truncated by channel migration, further suggest the Nooksack’s northward Holocene course through the Sumas Valley to the Fraser. Two much smaller streams, the Sumas River and Johnson Creek, now flow northward in the Sumas Valley within these oversized relict channels. The abundance and morphological newness of the remnant channels, Native American legend, and various radiocarbon dates indicate that the Nooksack River avulsed from the Fraser basin to its modern course to Bellingham Bay in the late Holocene.