This study uses drifter-based observations to investigate the role of wind and waves on spreading and mixing in the Fraser River plume. Local winter wind patterns commonly result in two distinct forcing conditions, moderate winds from the southeast (SE) and strong winds from the northwest (NW). We examine how these patterns influence the spreading and mixing dynamics of the plume. Under SE winds, the plume thins, spreads, and turns to the right (north) upon exiting the river mouth. Mixing is initially intense in the region of maximum spreading, but it is short-lived. Under NW winds, which oppose the rightward tendency of the plume, the plume remains thicker, narrower, and flows directly across the Strait with a lateral front on its northern side. Mixing is initially lower than under SE forcing but persists further across the Strait. A Lagrangian stream-normal momentum balance shows that wind and interfacial stress under NW conditions compress the sea surface height anomaly formed by the river discharge and guide the flow across the Strait. This reconfiguration changes spreading and mixing dynamics of the plume; plume spreading, which drives intense mixing under SE winds, is shut down under NW winds, and mixing rates are consequently much lower. Despite the initially lower mixing rates, the region of active mixing extends further under NW winds, resulting in higher net mixing. These results highlight that the wind, which is often a primary cause of increased plume mixing, can also significantly influence mixing by changing the geometry of the plume.Plain Language Summary Rivers transport sediment, pollutants, and nutrients from inland regions to coastal seas. Where rivers meet the ocean, freshwater flows over the ambient salty seawater, forming a river plume. The quantities that the river transports into the ocean are mixed into the seawater along with the freshwater, and so it is vital to understand this mixing process. While ocean surface waves might be the most striking visual feature of the coastal ocean, at the Fraser River mouth, south of Vancouver, Canada, we find that wind is a much more important influence on river plume mixing than waves. Wind can influence mixing by changing the geometry of the plume to either favor or discourage intense mixing occurring in the system. This is a result of the wind encouraging or discouraging plume spreading, which is the primary cause of mixing close to the river mouth. Ocean surface waves, despite being a visually striking feature of this system, do not play a large role in mixing the Fraser River plume. Thus, in order to correctly predict river plume mixing, we must take into account wind conditions near the river mouth, while waves are less important.

We use observations from the Quinault River, a small river that flows into an energetic surf zone on the West Coast of Washington state, to investigate the interaction between river and wave forcing. By synthesizing data from moorings, drifters, and Unmanned Aerial System video, we develop a conceptual model of this interaction based on three length scales: the surf zone width, L SZ ; the near-field plume length, L NF ; and the cross-shore extent of the channel, L C . The relationships between these length scales show how tidal variability and bathymetric effects change the balance of wave and river momentum. The most frequently observed state is L SZ > L NF . Under these conditions the surf zone traps the outflowing river plume and the river water's initial propagation into the surf zone is set by L NF . When the river velocity is highest during low water, and when wave forcing is low, L NF > L SZ and river water escapes the surf zone. At high water during low wave forcing, L C > L SZ , such that minimal wave breaking occurs in the channel and river water escapes onto the shelf. Based on the discharge, wave, and tidal conditions, the conceptual model is used to predict the fate of river water from the Quinault over a year, showing that approximately 70% of the river discharge is trapped in the surf zone upon exiting the river mouth.Plain Language Summary Small rivers are an important source of sediment, nutrients, and pollutants to the coastal ocean, but they are less well studied than their larger siblings. The coastal discharge from small rivers often meets large breaking waves in the surf zone. Our work investigates the effect of the large waves present at the mouth of one such river, the Quinault River in Washington State. We find that the fate of Quinault River water is determined by the relative importance of river, wave, and depth effects, all of which are modulated by the tide. When wave forcing dominates, river water is trapped in the surf zone. When river forcing dominates, river water escapes the surf zone. The difference in depth between the offshore channel and the rest of the beach can also allow river water to escape the surf zone by reducing the effect of the wave forcing. When we apply our conceptual model to 1 year of wave, river, and tide data, we predict that 70% of Quinault River discharge is trapped in the surf zone. The sediment and pollutants carried by this trapped river water can thus have important impacts on beach erosion, public health, and local ecology.Small river mouths are often unengineered, allowing surf zone wave breaking to occur near or at such outflows. In regions with higher population densities, engineered structures such as jetties are present at most river mouths, preventing wave breaking and the influence of surf zone dynamics on buoyancy and

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The effects of wind-driven whitecapping on the evolution of the ocean surface boundary layer are examined using an idealized one-dimensional Reynolds-averaged Navier-Stokes numerical model. Whitecapping is parameterized as a flux of turbulent kinetic energy through the sea surface and through an adjustment of the turbulent length scale. Simulations begin with a two-layer configuration and use a wind that ramps to a steady stress. This study finds that the boundary layer begins to thicken sooner in simulations with whitecapping than without because whitecapping introduces energy to the base of the boundary layer sooner than shear production does. Even in the presence of whitecapping, shear production becomes important for several hours, but then inertial oscillations cause shear production and whitecapping to alternate as the dominant energy sources for mixing. Details of these results are sensitive to initial and forcing conditions, particularly to the turbulent length scale imposed by breaking waves and the transfer velocity of energy from waves to turbulence. After 1-2 days of steady wind, the boundary layer in whitecapping simulations has thickened more than the boundary layer in simulations without whitecapping by about 10%-50%, depending on the forcing and initial conditions.