Wind Effects on Small‐Scale River and Creek Plumes

In contrast to large river plumes, Coriolis effects are weak, and inertia is quickly depleted so that the fate and structure of small‐scale plumes are more sensitive to tide and wind. Advected alongshore by reversing tidal currents in absence of wind forcing, small buoyant plumes are persistently deflected downwind in presence of alongshore winds and exhibit little tidal variability. The effect of different upwelling/downwelling winds on buoyant outflows ∼10 m3 s−1 is explored. With increasing wind, tidal variability decreases, as does asymmetry in plume characteristics—for strong winds upwelling/downwelling plume structure is similar as the plume is retained closer to the shore. Wind forcing is exerted directly by wind stress on the surface of the plume and indirectly by wind‐driven currents that deflect the upwind boundary of the plume. While inertia and buoyancy dominate the inner plume, and wind dominates the outer plume, the mid‐plume responds to an interaction of wind and buoyancy forcing that can be indexed by a Plume Wedderburn Number Wpl (wind stress vs. density gradients): for weaker winds (Wpl < 1) surface stress enhances stratification through straining, lengthening the reach of low‐salinity waters, whereas for stronger winds (Wpl > 1) surface stress mixes the plume vertically, shortening the reach of low‐salinity waters. However, dilute plume waters extend furthest in strong winds, passively advected several kilometers downwind. Shoreline exposure to outflow transitions from a quasi‐symmetrical tide‐averaged zone of impact under zero‐wind to a heavily skewed zone with persistent weak wind and a one‐sided zone for strong wind. Plain Language Summary Compared to large river plumes, outflow from small rivers and mountainous streams is more sensitive to tides and winds because of the weak Coriolis effect and quickly reduced inertia. Alongshore (upwelling/downwelling) winds carry these small plumes in their direction. We use a numerical model to study the effect of these upwelling/downwelling winds on plumes spreading from small rivers with discharge rates of 10 m3 s−1 or less. Increasing wind reduces tidal fluctuations in plume patterns such that with strong winds the plume spreads similarly for upwelling and downwelling winds as it remains close to the shore. Wind affects the plume surface directly and the upwind‐plume boundary indirectly via wind‐driven currents. Inertia and buoyancy control the inner plume while wind and buoyancy control the mi