Hydrography of the Russian River Estuary - Sonoma County Water ...

Figure 3.35. Time-series of potential energy anomaly ϕ at primary stations.
During the muted tidal conditions, prior to closure on September 7, an increase in water
column stability was observed at all stations as stratification built up. This happened in
spite of a net loss of salt from the estuary over this week (see Section 3.5), evidently
due to the outflow of freshwater near-surface which resulted in lower surface salinities
and increased stability throughout the estuary. Velocity data show a near-surface
(freshwater) jet extending only 20-30 cm into the water column (see Figure 8.3 in
Behrens & Largier 2010), which could account for the advective export of salt from this
uppermost layer and the consequent sharpening of the pycnocline (and higher water
column stability).
During the September-October closure event, water column stability continued to
increase for all stations, due to increasing water depth as well as the intrusion of saline
waters near-bottom at inner estuary stations and the intrusion of low-salinity waters
near-surface at outer estuary stations. The increase in stability at the Mouth station is
least, and levels out after about 3 weeks. This is due to the slow decrease in the salinity
of deeper waters (Figure 3.36), countering the increase in ϕ due to increasing h. In
contrast, near-bottom salinity at Paddyʼs Rock remains unchanged below 5m NGVD,
and little changed at all depths below 3m – the height of the shoal separating this
station from the Mouth station (cf., Largier & Taljaard 1991, Largier & Slinger 1991).
Whether due to vertical mixing or seepage through the bar, the changing vertical
distribution of salinity near the Mouth station may have led to a decrease in stability if
the mouth remained closed longer – and, ultimately, the stability may reduce enough for
the water column to be mixed by winds. This could flush out the lower-layer hypoxia and
the possibility of this happening needs further attention.
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