The Russian River Estuary: Inlet Morphology, Management, and Estuarine Scalar Field Response

This dissertation explores three areas: (1) Variability of inlet closure frequency, (2) response of the estuarine salt field to inlet closure, and (3) the influence of inlet management on stratification in the estuary. These questions are addressed by examining the Russian River Estuary (RRE), a prototypical bar-built estuary in Northern California. This work builds from existing studies of the site by the author, leveraging a uniquely extensive set of in situ observations of daily inlet behavior with additional field and numerical modeling work. The aim is to use these tools to provide a more complete picture of the combined function of the inlet and estuary than is currently present in the literature.;Inlet closure frequency is examined using over 60 years of daily closure records detailing the behavior of the Russian River mouth. Inlet morphological behavior is shown to be a response to processes which act to scour the inlet (driven by tides and river flows) and processes that act to deposit sand in the inlet (driven by waves). The observed time-dependent closure pattern is shown to be well-described as a response to tidal, weather-related, seasonal and interannual processes. A parametric model is developed to predict closure events in the short- and seasonal-term scales, and is shown to improve on existing models. Lastly, closure records from the nineteenth and twentieth centuries are compared, showing that inlet closures have become shorter and more sporadic over time at this site. Long term shifts such as this are examined within the context of management practices and climate variability.;The evolution of the salt field is tracked during several inlet closure events in 2009 and 2010 using repeated boat-based conductivity temperature depth (CTD) surveys, stationary sondes and upward facing acoustic doppler current profilers (ADCPs). The motion of the intruding salt front is in general well-studied in relatively deep estuaries with gradually varying bathymetry. However, in bar-built estuaries such as the RRE, the mechanisms of upstream salt transport are more obscure, since these systems are often associated with sharply-varying bathymetry resulting from extensive meandering and high stream gradients. The field data are used to show that the salt front movement persists in the estuary despite these characteristics. Empirical orthogonal functions and a horizontal Richardson Number are employed to link the salt transport into the inner estuary to buoyancy-driven currents which move epilimnetic salt water upstream along the top of the pycnocline when diurnal winds are not active. Internal waves are also present, but are limited by the bathymetry in the extent to which they transport salt upstream. Volumetric capacity of the pools upstream of the front and turbulence caused by diurnal winds are shown to be the main impediment to the salt front advance.;The impacts of inlet management on stratification in the estuary are assessed by examining two practices: (1) Allowing the inlet to remain closed and (2) enforcing perched conditions by cutting a one-way supratidal outflow channel over the beach. The ramifications of the latter are presently unknown. A numerical model is used to examine the different outcomes achieved during hypothetical two-week closure and overflow events at the RRE. The model uses a two-dimensional (x-z plane) representation that emphasizes the vertical salt stratification in the estuary. The modeling results show that having an overflow channel only several meters above the pycnocline results in strengthened stratification when winds are applied, while inlet closure leaves stratification mostly intact. Long-term changes in estuary salt mass are well-described as a function of the hydraulic gradient through the beach during closure, and as a function of vertical diffusion of salt during perched overflow. A ratio of the magnitudes of seepage flow out of the beach and vertical diffusive flow of salt across the pycnocline is a good descriptor of which process will dominate the long-term evolution of salt in the estuary. Boat-based field data confirm the model results, suggesting that the two management states present divergent outcomes in the estuary. (Abstract shortened by UMI.).