Rapid riparian vegetation development and channel stabilization linked to glacier retreat within a large transboundary watershed

Abstract

River floodplains are dynamic mosaics of aquatic and riparian habitats maintained by flow regimes and sediment processes, but these floodplains may be rapidly shifting with climate change. Specifically, previous work has highlighted that glacier retreat can be associated with river channel and floodplain change. Within a large and glacially influenced watershed, the Taku Watershed of northwestern British Columbia and southeastern Alaska, we used remote sensing and Geographic Information System (GIS) to quantify four decades of annual floodplain habitat composition and river channel patterns in 16 subwatersheds that naturally varied in their glacier coverage and other landscape (e.g., floodplain elevation, slope) and climate variables (e.g., temperature, precipitation). We discovered rapid development of floodplain vegetation and increased habitat stability in subwatersheds with higher glacial coverage, and a lack of directional change in other subwatersheds. Floodplain vegetation coverage almost doubled over 38 years in subwatersheds with the highest glacier coverage. Focal analyses revealed that river channel structure also shifted in glacierized subwatersheds, with decreases in habitat turnover rate, water occupancy and some evidence for decreasing levels of channel braiding. A large landslide (1.5 km runout, 130 ha) during the study period caused a local disturbance to one of the floodplains, but with little immediate broader impacts at the scale of the entire floodplain. Collectively, climate change and associated glacier retreat are leading to the rapid transformation of floodplain ecosystems, with major implications for the habitat of important species such as migratory salmon.