Estuarine Circulation through Hands on Learning

Parker MacCready from the UW School of Oceanography demonstrates injecting dyed fresh or saltwater in various depths and locations of the "Puget Sound"

Tucked away in a beautiful brick building on a canal at the University of Washington (behind a free standing whiteboard in the University of Washington's Old Oceanography Building) is an impressive and powerful visualization tool. A to-scale model of the Puget Sound was built in 1950 by the School of Oceanography and has been used to research and teach tidal and circulation characteristics in our complex estuary.

Water enters the Puget Sound from the Pacific Ocean through the Strait of Juan de Fuca, is mixed up in Admiralty Inlet, then pours over shallow sills into four major fjord-like basins.  Some of these are well mixed and regularly circulated while others are stagnant and stratified.  The system is further complicated by tides, freshwater input from rivers the seasonal appearance of cold, salty water from upwelling along the coast.

Dyed freshwater injected at the "Duckabush River" output forms eddies on the surface as it slowly moves north out of "Hood Canal"

This model is so lasting because it attempts to take some of these complications into account.  For example, a tide machine near the model's "ocean" moves water in and out of the system, taking about one minute to complete each tidal cycle.  Tubes beneath the model constantly trickle freshwater at roughly the same rate and location as main rivers.  The bathymetry of the Sound is represented and exaggerated to allow for the viscosity of the water at this scale.

Students are able to inject either fresh or saltwater mixed with blue dye anywhere in the system and observe what happens.  This week a group of students noticed that fresh water stayed near the surface and tended to move out of the system, toward the "ocean," while dense salt water sank.  The deep salty water would then slowly move into the system or get stuck in various deep pockets like in Dabob Bay, Carr Inlet or just west of Deception Pass.  They also noticed a large difference between the different freshwater inputs.  A few drops of dyed water on the "Skagit River" dissipated into Whidbey Basin almost immediately while dye at the "Snohomish River" input remained for the entire class period.

A bird's eye view over "Whidbey Island" shows fresh water from the "Skagit River" quickly dispersing into the basin while dense salt water is trapped in a deep pocket just outside of "Deception Pass"