SEDIMENT DELIVERY AND DISPERSAL FROM THE COPPER RIVER, ALASKA, FOLLOWING RECORD SNOWFALL

The 2011-2012 winter produced the highest recorded cumulative snowfall for the Copper River Basin in Alaska since records began in 1950. The subsequent spring melt resulted in high freshwater and sediment discharge in the Copper River with recorded suspended-sediment concentrations reaching as high as ~1.8 g/L. The Copper River supplies a variety of vital nutrients that support the primary production in the biologically abundant waters of the Gulf of Alaska and Prince William Sound. Elevated water discharge combined with high sediment concentrations can potentially alter delivery mechanisms and dispersal patterns for this large and ecologically important subarctic river. Observations of water properties and suspended-sediment concentrations in the Copper River and the adjacent shelf were made during a high discharge event in the summer of 2012. On June 28^th and 29^th an instrumented profiling tripod was deployed at 55 individual stations in four distinct transects that extended from within the river mouth out onto the shelf and two shore-normal transect bounding the river mouth. The plume is strongly stratified with a salinity of ~1.5 ppt upstream of a shallow bar at the mouth, a plume thickness of ~4.9 m seaward of the bar thinning to ~1.5m at 3.5 km offshore. Suspended-sediment concentrations in the river reach ~1.8 g/L and decrease to ~0.02 g/L at the seaward extent of the visibly turbid surface plume. The cold temperatures combined with high sediment concentration could lead to a near-bottom cross-shelf gravity flow. The preliminary salinity and density cross-section contours indicate a near-bottom component in three of the four transects with a lower salinity water layer extending along the bottom as far as ~3.4 km offshore. The low salinity water causes a low density anomaly equivalent to suspended-sediment concentrations of ~ 13 g/L. Rapid settling from the plume as it exits the mouth and the low salinity bottom water suggests the potential for hyperpycnal flow and suggests the importance of near-bottom fluxes during high discharge events. Furthermore, climatic alterations as a result of global warming could increase the frequency and intensity of these conditions in high-latitude river systems.