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

All water properties are subject to the same patterns of transport and mixing, circulating
cooler saline seawater and warmer freshwater through the estuary. However, the mass
balances that control the evolution of properties also include non-conservative terms
(such as the surface warming of water) that differ between properties. Thus salinity
patterns do not match temperature, nutrient or dissolved oxygen patterns and one may
find habitats with any combination of property values. As there is neither source nor sink
for salt dissolved in estuarine waters, salinity is a conservative property and provides
the clearest view of transport and mixing patterns in the estuary. However, the
distribution of temperature, nutrient, or dissolved oxygen in the estuary is not simply
explained by the mixing of freshwater with seawater (as is true for salinity), thus an
explanation of distributions also requires knowledge of the spatial pattern in processes
such surface heating, respiration, and photosynthesis.
3.3.1 – Spatial distribution of salinity
The distribution of salt in the Russian River estuary is comprised of vertical and
longitudinal structures. The longitudinal structure is due to seawater mixing in from the
mouth while freshwater flows in at the landward boundary. The vertical structure is due
to the large density difference between seawater and freshwater, so that saltier waters
flow landward beneath seaward flowing low-salinity waters (see plots in Behrens &
Largier 2010).
For convenience, one may identify 3 hydrographic zones in the Russian River estuary:
(i) the outer estuary, up to Sheephouse Creek, is characterized by strong tidal currents
and associated fluctuations in salinity; (ii) the mid-estuary, from Sheephouse Creek to
Heron Rookery, is characterized by weaker and more variable tidal fluctuations in
salinity and the trapping of saline water in deeper pools for extended periods of time;
and (iii) the inner estuary, landward of Heron Rookery, is often completely fresh, but
saline waters intrude under specific tidal conditions or during closure events and
pockets of salinity may be retained in deep pools following such an event.
Vertical structure in salinity is strong in the Russian River estuary. While the water
column may mix during strong tidal flows, the continual inflow of river water ensures that
stratification is rapidly re-established as currents weaken hours later or as ebb-tide
straining helps to counter vertical mixing. During neap tides, or as the mouth constricts
tidal flows, vertical stratification will persist unbroken, even at stations near the mouth.
The wind may also deform or mix the surface freshwater layer as surface stress is
imposed directly on this thin low-salinity layer – but, as for the tide, as soon as the wind
weakens, near-surface stratification is re-established.
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