TRANSLATING BED WAVES IN RESPONSE TO CLIMATE-DRIVEN VARIABILITY IN SEDIMENT SUPPLY FROM MOUNT BAKER, WASHINGTON STATE

Sediment yields from glaciated basins are broadly expected to be influenced by recent trends in climate and glacier extent, but links between climate variability, sediment yield and subsequent channel response have often been difficult to identify. In the Nooksack River Basin (Washington State, USA), which drains glaciated terrain on the Mount Baker stratovolcano, long-term records of channel aggradation and incision were inferred from changing stage-discharge relations at U.S. Geological Survey stream gages. Vertical channel change at seven gages in the basin show a strong, but lagged, correlation with regional climate trends. The lag between climate signal and channel response increased monotonically from 20 to 65 years as a function of downstream distance, indicating that headwater supply signals were transmitted down the river as a broad translating bed wave. The velocity of the wave ranged from one to four kilometers per year and was positively correlated with channel slope. Climate-driven variations in headwater sediment supply then appear to be readily transmitted down the Nooksack, in the form of 0.5-1 m variations in bed elevation, and have been the dominant signal in records of vertical channel change over the past century. Channel change and sediment yields in a number of other regional basins suggest that these processes are not unique to the Nooksack, but may be shared across physiographically-similar basins and synchronized by shared climate and glacier trends. The 120-year history of climate, sediment and channel change shows a strong signature of multi-decadal variability associated with the Pacific Decadal Oscillation, including a period of notably low sediment supply from 1950 to 1980. These results provide a historic context for recent change, and help constrain what physical processes link climate to sediment supply. A more complete understanding of those physical processes are needed to better forecast sediment supply changes under continued warming