Episodic closure of the tidal inlet at the mouth of the Russian River — A small bar-built estuary in California

Bar-built estuaries with relatively small (cross sectional area < 100 m2) and shallow tidal inlets are widespread in Mediterranean climates and along wave-exposed coasts. While similarly important to coastal sediment balances and estuarine ecosystems and more numerous than larger inlet systems, they suffer from a relative lack of understanding. This is especially true regarding the process of inlet closure, when the channel is filled with sediment deposited by wave-driven processes. Mouth closure is of growing concern owing to changes in inlet behavior in many systems in response to human influences. The use of many existing conceptual and quantitative models for closure is precluded by rapid morphological change, unsteady freshwater inputs, and difficulties in scaling well-established inlet relationships in these smaller systems. In this paper, over 60 years of daily closure records are analyzed for the Russian River mouth, a prototypical bar-built system in northern California. We show that the time-dependent closure pattern can be explained by an interaction of processes that act to scour the inlet (driven by tides and river) with those that act to deposit sand in the inlet (driven by waves). Tidal, weather-related, seasonal, and interannual cycles are observed. We improve an existing parametric model to evaluate these separate influences and show that it works well for predicting closure events at both the tidal and seasonal scales. Finally, we compare our data with closure records from the nineteenth and twentieth centuries to show that inlet closures have become shorter and more sporadic over time at this site and to identify anthropogenic drivers of this observed change.