Landward Propagation of Saline Waters Following Closure of a Bar-Built Estuary: Russian River (California,USA)

We investigate the evolution of the salt field in a bar-built estuary after the tidal inlet is closed by sediment, isolating the estuary from the ocean. We show that seawater trapped by inlet closure in the Russian River Estuary, CA, undergoes a two-stage landward intrusion process that leads to widespread salt stratification throughout the estuary. This salinity intrusion extends to distances of several kilometers from the beach—into the “inner estuary” that is separated from the “outer estuary” by shallow sills and typically devoid of saline waters during tidal conditions when the mouth is open. We describe landward movement of saline waters during six closure events in 2009 and 2010, based on repeat boat-based conductivity-temperature-depth (CTD) surveys and bottom-mounted acoustic Doppler current profilers (ADCPs). While sills block the initial landward motion of dense saline waters due to gravitational adjustment (first stage of intrusion), these same sills facilitate a wind-induced, one-direction valve mechanism through which saline waters are pumped into the inner estuary. Saline waters that crest the shallow sill can drain into deeper pools in the inner estuary as a pulsed gravity current (second stage of intrusion). We use empirical orthogonal function (EOF) analysis to identify an internal seiche in the outer estuary that results in uplift of pycnocline waters during the night at the boundary to the inner estuary. EOF analysis of inner estuary currents and a horizontal Richardson number are used to suggest that nocturnal gravity current events in the inner estuary (beyond the blocking sill) occur as pulses initiated by the internal seiche in the outer estuary.