Fluvial impacts on tidal range, salinity intrusion, and the fresh–saltwater interface.

The fresh–saltwater interface in estuaries is a region of increased biological activity that is influenced by a variety of hydrodynamic processes. For example, the salinity intrusion length and salinity gradient in estuaries are controlled by horizontal density gradients and vertical mixing, driven to first order by the tides, but also by the strength of the river flow. Despite being strongly linked, tidal and river inputs are traditionally decoupled when studying the salinity field analytically. However, since tidal amplitudes can be damped by O(10%) through elevated friction due to tidal–fluvial interactions, analytically separating tides and rivers can overestimate tidal mixing and distort the response of the salinity field to external forcing during periods of high flow. We develop a 2-D semi–analytical model to investigate the effect of tidal–fluvial interactions on estuary mixing and the salinity field. The model simulates gravitational circulation, internal tidal asymmetry and river flow to estimate tidally averaged, along-channel salinity distributions under various tidal and river conditions. Results demonstrate that under appropriate conditions, tidal damping re-enforces gravitational circulation and internal tidal asymmetry and increases salinity intrusion. Including tidal–fluvial interactions suggests that spring/neap transitions in vertical mixing and salinity intrusion are variable for the same oceanic tidal conditions and shift seasonally with river flow and the variation in tidal damping. Since many systems experience large annual variability in river flow this effect is likely a global phenomenon, though estuaries that experience strong internal tidal asymmetry may be more susceptible since asymmetry in vertical mixing is sensitive to modulations of tidal amplitudes.