Freshwater Discharge and Sediment Dispersal on the Alaskan Beaufort Shelf

David Ralston, Applied Ocean Physics & Engineering
Rocky Geyer, Applied Ocean Physics & Engineering


Arctic Research Initiative
2011 Funded Project


We propose a field investigation of the dynamics of river outflows into the Arctic Ocean, specifically in the shallow waters adjacent to a river mouth where the outflow impinges on ice.  The study is motivated by the rapidly warming climate of the high Arctic, which may be increasing the delivery of sediment and organic matter to the ocean via Arctic rivers.  Arctic river deltas are distinct from their lower latitude counterparts due to the presence of sea ice in the receiving waters during the spring freshet.  The sea ice reduces the velocity of the outflow, disconnects the plume from mixing by the wind, and impounds fresh water.  As a result, sediment carried by the river outflow deposits and accumulates near the river mouth.  The sediment trapping and minimal remobilization by tides and wind make Arctic deltas much shallower than deltas in ice-free environments.  During the short but intense spring floods that deliver sediment to the coastal Arctic, the ice at the river mouth is rigidly attached to the shore, or landfast, and the outflow goes both under and over this 2-m thick sheet of ice.

Our scientific goal is to characterize the buoyant plume and associated suspended sediment flux as the river outflow enters the coastal ocean and disperses.  To accomplish this goal, we propose a novel measurement array that samples water properties in the river plume both above and below the landfast ice by extending sensors above and below the ice surface.  To link the near surface river plume to the underlying oceanic water, we will collect high-resolution profiles of velocity and acoustic backscatter (used to quantify suspended sediment transport) from a bottom mounted tripod over the full water column below the ice.  The instruments will be deployed in the late winter and recovered in late summer, if they survive the river flooding and ice break-up.  The data from all of the instruments will be telemetered via the Iridium satellite link back to our lab to ensure maximum data recovery.  We will also survey the delta region via helicopter during the tumultuous period of ice break-up, to characterize the spatial structure of the river outflow and provide context for the moored time series.

Our field site is the Colville River estuary and nearby coastal region of the Alaskan Beaufort Shelf.  The Colville is the biggest river draining Alaska's North Slope, and among Arctic rivers it may be particularly susceptible to changes due to warming because its entire watershed is in the zone of continuous permafrost.  Compared with mid-latitude and tropical rivers, this type of river system has low sediment yield and limited transport of particulate organic carbon.  However as the permafrost melts, the loading of sediment and organic matter is expected to increase dramatically.  Our study of the mechanisms of river outflow and sediment dispersal across sea ice addresses one the critical processes that will affect our understanding of how the Arctic responds to shifting climatic conditions.