CLIMATE SINGALS IN STREAMBED ELEVATIONS OF GLACIAL RIVERS IN SOUTHCENTRAL ALASKA

Glaciers and their outflows are particularly sensitive to changes in climate. Glacial rivers respond to meteorological forcings ranging over diurnal to secular timescales and in turn the magnitudes of discharge, sediment load, and stream power in these rivers also respond. Short-term response includes periods of scour and fill or bank erosion, long-term response includes aggradation or degradation and lateral channel migration. A primary driver to these forcings in southcentral Alaska are phase shifts of the Pacific Decadal Oscillation (PDO). The PDO is a regional pattern of climate variability in the North Pacific with regimes typically persisting for decades. The warm PDO phase is characterized by higher than average summer and winter temperatures relative to the cold phase. The PDO shifted from cold to warm in 1976 and this phase persisted until 1998, when it shifted back to the cold phase. The PDO reversed back to warm from 2002-2005 before returning to the cold phase. It is this last shift from warm to cold that is evident in streambed-elevation datasets. Repeat cross sections and continuous streambed-elevation data collected in front of bridges spanning glacial rivers in southcentral Alaska from 2003-2008 show a change in average and minimum elevations from 2003-2005 to 2006-2008 that is attributed to a phase shift of the PDO. Average summer temperatures were 1.5° C higher relative to the cold phase, resulting in higher average discharge at three glacial rivers during the summer months of the most recent warm phase. Increased discharge resulted in greater cross-sectional area and greater stream power, which in turn scoured streambeds to lower elevations. The average minimum bed elevation at the Knik River was 2.5 meters lower during the warm phase, exposing pier foundations six times. Average annual temperatures in Alaska are projected to rise 2 to 4° C by the middle of this century. Increases in discharge and the resultant streambed scour can be expected for glacial rivers with this magnitude of temperature increase. The long-term response of these rivers will be characterized by increases in channel scour and lateral channel migration.