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

This diurnal cycle in winds results in a setup of water level towards the back of the
estuary during the afternoon as a result of the movement of near-surface waters that are
directly forced by surface stress. Under some circumstances a pressure-driven return
flow may develop at depth, beneath the direct effect of the surface stress. Nevertheless,
the water level setup will remain as long as the wind blows. However, as the wind
weakens, a landward flow will develop as the setup relaxes. This wind-driven seiche is
clearly seen in water level data (Figure 3.16): around noon one can see the mouth water
level dropping while the water level at Freezeout Island increases. A weaker increase in
water level is also seen at Heron Rookery and a barely perceptible increase at Willow
Creek (which must be near the pivot point for the water surface). As the sea breeze
dissipates the slope in the water level relaxes, and may even tilt the other way during
the night (e.g., early hours of 4 October). This forced seiche recurred daily. This wind
forcing is also active during tidal periods, but the signal is swamped by tidal motions.
Figure 3.16. Diurnal fluctuations in water level at 4 stations during inlet closure.
The strength of transport associated with this seiche may be calculated either from
water level data or from ADCP current velocity data. Water level measurements and
estuary bathymetry were used to estimate the volume of water displaced upstream – the
volume of water lost from the outer estuary should be equivalent to the volume gained
by the inner estuary. At the same time, an estimate of the volume displaced past Heron
Rookery is obtained by multiplying velocities measured at each depth by the channel
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