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

Executive Summary
Circulation, stratification and water properties were monitored during late summer 2009
in the Russian River estuary– a dry year in which river flow was lower level than normal.
Attention is directed at conditions in a month-long closure event in September-October.
A dense lower layer of high-salinity water was trapped in the estuary when the mouth
closed, with stability increasing over time as well as an expansion of stable stratification
as saline waters intruded into the inner estuary. Within a week of strong stratification
being established, the near-bottom waters became hypoxic. At mid-depth penetration of
light led to photosynthesis and a stable layer in which oxygen levels were high during
the day. At similar depths, thermal radiation was also trapped and water temperatures
were greatest. The thin surface layer was well mixed and in equilibrium with the
atmosphere in terms of both dissolved oxygen and temperature. Stratification and deepwater
hypoxia persisted until tidal action returned with opening of the mouth in October.
This report is preceded by a data report (Behrens & Largier 2010), in which all field data
are plotted and details are provided on instrument deployments. In this report, core
sections address (i) water budget and seepage analysis, (ii) tidal and diurnal currents,
(iii) hydrographic structure – salinity, temperature, dissolved oxygen, (iv) stratification
and water column stability, (v) salt and dissolved oxygen budgets.
Central to this study and the value of the Russian River estuary as habitat for juvenile
salmon is the strong stratification that develops due to trapping of a salt layer in the
estuary when the mouth closes. The salt-stratified closed estuary is non-tidal and a
wind-driven diurnal seiche takes on particular importance in the horizontal redistribution
of salt as well as in raising the possibility of vertical mixing and eventual breakdown of
the stratification. Breakdown of stratification appears essential for re-oxygenating the
hypoxic bottom waters and reducing mid-depth temperatures. A significant apparent
loss of estuary water through the sand barrier (~60cfs) is important in flushing the
surface layer (residence time order 10 days), but it plays only a minor role in reducing
the salinity of the lower layer within 1-2 km of the barrier beach.
The most significant advances in future understanding in support of improved
management of the estuary are likely to come from (i) linking water property
distributions to salmon habitat value and extent, (ii) assessment of the extent of hypoxia
immediately following breaching after a long closure, (iii) assessment of turbulence,
vertical mixing and the potential for breakdown of stratification, (iv) a fuller quantification
and understanding of the diurnal wind-driven seiche, (v) a fuller quantification and
understanding of seepage losses through the sand barrier, and (vi) a fuller quantification
and understanding of berm overflows and wave overwash.
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