Dynamics of Willapa Bay, Washington: a highly unsteady, partially mixed estuary

N. S. Banas, B. M. Hickey, P. MacCready, J. A. Newton

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Abstract

Results from 3 yr of hydrographic time series are shown for Willapa Bay, Washington, a macrotidal, partially mixed estuary whose river and ocean end members are both highly variable. Fluctuating ocean conditions - alternations between wind-driven upwelling and downwelling, and intrusions of the buoyant Columbia River plume - are shown to force order-of-magnitude changes in salinity gradients on the event (2-10 day) scale. An effecticve horizontal diffusivity parameterizing all up-estuary salt flux is calculated as a function of riverflow: results show that Willapa's volume-integrated salt balance is almost always far from equilibrium. At very high riverflows (the top 15% of observations) the estuary loses salt, on average, while at all other riverflow levels it gains salt. Under summer, low-riverflow conditions, in fact, the effective diffusivity K is large enough to drive a net increase in salinity that is 3-6 times the seaward, river-driven salt flux. This diffusion process is amplified, not damped, by increased tidal forcing, contrary to the expectation for baroclinic exchange. Furthermore, K varies along the length of the estuary as ∼5% of the rms tidal velocity times channel width, a scaling consistent with density-independent stirring by tidal residuals. To summarize Willapa's event- and seasonal-scale variability, a simple diagnostic parameter space for unsteady estuarine salt balances is presented, a generalization from the Hansen and Rattray steady-state scheme.

Original languageEnglish
Pages (from-to)2413-2427
Number of pages15
JournalJournal of Physical Oceanography
Volume34
Issue number11
DOIs
Publication statusPublished - 1 Nov 2004

Keywords

  • density (specific gravity)
  • diffusion
  • hydrographic surveys
  • oceanography
  • rivers
  • salts
  • tides
  • time series analysis
  • wind effects
  • fluctuating ocean conditions
  • tidal forcing
  • Willapa Bay, Washington
  • wind-driven upwelling
  • estuaries
  • estuarine dynamics
  • mixing
  • river discharge
  • salinity
  • North America
  • United States
  • Washington [United States]
  • Western Hemisphere
  • world

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