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

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.

Automated summary

Hydrographic measurements were used to investigate seasonal variability in the strongly stratified Duwamish River Estuary across a 20-fold seasonal range of river discharges. The effect of river discharge on salt wedge length, stratification, pycnocline thickness, and intratidal differences in salinity structure was evaluated. The salt wedge decreases in length and becomes more stratified as river discharge increases.