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

4. Discussion
Circulation, stratification, and water properties were monitored in the Russian River
estuary during late summer and fall 2009 – a dry year in which river flow was managed
at a lower level than normal. Specific attention is directed at conditions during a monthlong
closure event in September-October. A dense lower layer of high-salinity water is
trapped in the estuary when the mouth closes, with water column stability increasing
over time as well as an expansion of stable stratification into the inner estuary, which is
devoid of salt during periods of higher river flow. Within a week of strong stratification
being established, the near-bottom waters become hypoxic (i.e., below the 1-percent
light level). At mid-depth, however, penetration of light leads to photosynthesis and a
stable layer in which oxygen levels are super-saturated during the day. At similar
depths, thermal radiation is also trapped and water temperatures are maximum. A thin
surface layer is typically well mixed and in equilibrium with the atmosphere in terms of
both dissolved oxygen and temperature. Stratification and deep-water hypoxia persist
until tidal action returns with opening of the mouth in October.
While dense waters may be trapped beneath a pycnocline even during times when the
mouth is open, surface waters move through the estuary quickly when the mouth is
open (residence times ~ 1day). Similarly mixed waters are rapidly removed from the
estuary through the action of tides. However, as tidal action weakens due to a reduction
in mouth conveyance, mid-depth and near-bottom saline waters become resident in a
stratified estuary. Residence time typically exceeds the period in which muted tides are
observed and the mouth closes with these waters still retained in the estuary – thus
residence time becomes closure time (over a month in September-October 2009). As
stratification develops later in the inner estuary during closure, saline waters are only
trapped there later and residence in this location is shorter (still long enough for deoxygenation)
– but these saline waters were already resident in the estuary and have
only shifted location. Meanwhile, surface waters will continue to move through and out
of the estuary as long as the bed of the mouth channel (or the crest of the berm) is
below the estuary water level. This “overflow” state has been observed at the mouth of
the Russian River, but this is not common, nor does it persist for more than a few days
(PWA, 2010). Even when closed, water is lost through the sand barrier (up to a rate of
about 60cfs later in the September 2009 closure). These waters are primarily lost from
above the pycnocline (Section 3.4.2), implying a residence time for near-surface waters
of order 10 days (calculated from volume/seepage rate). While this weak throughflow
entrains some water from the upper pycnocline, water beneath the pycnocline is
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