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Seasonal Changes in Structure and Dynamics in an Urbanized Salt Wedge Estuary

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Abstract

Hydrographic measurements of salt wedge structure and dynamics spanning a 20-fold seasonal range of river discharges are used to investigate the seasonal variability in the strongly stratified Duwamish River Estuary. The effect of river discharge on salt wedge length, stratification, pycnocline thickness, and intratidal differences in salinity structure are evaluated. The salt wedge decreases in length and becomes more stratified as river discharge increases. The ebb and flood responses to increasing discharge are markedly different; the flood phase structure shows little seasonal dependence, while the ebb structure changes due to an internal hydraulic response that is strongest at two severe lateral constrictions. This response varies with the net barotropic forcing, which is controlled by river discharge. The asymmetry between the structure on ebb and flood is explained using a two-layer hydraulic framework, although spatially, tidally, and seasonally variable vertical mixing also modifies the structure. As a result of the persistent stratification and dominance of hydraulic dynamics, channel constrictions influence circulation and transport during ebb tides. These dynamics also generate a flood/ebb asymmetry in salt wedge structure and circulation that is modulated seasonally through discharge. We hypothesize that these dynamics lead to a seasonally modulated residual circulation.

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Acknowledgments

The authors acknowledge that the Duwamish River runs through the ancestral lands of the Duwamish Indian Tribe and honor the traditional land and water of the Coast Salish Peoples on which this work took place. We are grateful for the many, many volunteers who provided help in the field, in particular, P. Clayton, J. Wilson, the UW EFM group, and 36 ambitious undergraduate fluid dynamics students. We also thank the 2012 FHL Estuarine and Coastal Fluid Dynamics class, in particular P. MacCready, W.R. Geyer, K. Arnott, J. Lopez, A. Tabatabai, S. Warner, and M. Williams. We also thank A. Ogston, C. Nittrouer, J. Thompson, A. Jessup, and Gravity Environmental LLC for the use of vessels and equipment, and the South Park Marina and Shumate Family for the use of their facilities. Larry Armi provided valuable input on the two-layer hydraulics. Additionally, we thank three anonymous reviewers who were instrumental in improving this manuscript. This research was supported by the University of Washington Royalty Research Fund, the Sea-Bird Electronics Student Equipment Loan Program, and the Amherst College Graduate School Fellowship Program. ARHD was supported by the Allan and Inger Osberg Professorship.

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Authors and Affiliations

  1. Department of Civil and Environmental Engineering, University of Washington, 201 More Hall, Seattle, WA, 98195-2700, USA

    Margaret A. McKeon & Alexander R. Horner-Devine

  2. Integrative Oceanography Division, Scripps Institution of Oceanography, UC San Diego, 9500 Gilman Drive #0206, La Jolla, CA, 92093-0206, USA

    Sarah N. Giddings

Authors
  1. Margaret A. McKeon
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  2. Alexander R. Horner-Devine
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  3. Sarah N. Giddings
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Correspondence to Alexander R. Horner-Devine.

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Communicated by Arnoldo Valle-Levinson

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McKeon, M.A., Horner-Devine, A.R. & Giddings, S.N. Seasonal Changes in Structure and Dynamics in an Urbanized Salt Wedge Estuary. Estuaries and Coasts 44, 589–607 (2021). https://doi.org/10.1007/s12237-020-00788-z

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  • DOI: https://doi.org/10.1007/s12237-020-00788-z

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