This chapter considers interactions between different forcings. It first describes the interaction between tidal currents and density gradients at intratidal (within one tidal cycle) time scales. One outcome of this interaction is the phenomenon known as tidal straining. The chapter continues with the treatment of intratidal variations of density that can also result from the interaction of density fields with tides and bathymetry. Subsequently, the chapter presents a description of the interaction between tides and density gradients at subtidal time scales, that is, at periods greater than one tidal cycle. The chapter then describes how advective accelerations from tidal currents can interact with density gradients to modify residual flows. It follows with a description of the competition between tidal stresses and density gradients in driving residual flows. It then deals with the competition between density gradients and wind stresses, to later add tidal forcing. The chapter then includes the influence of river discharge on estuarine circulation. The last two subsections present salt (or solute) budgets and their linkage to hydrodynamics and approaches to study saltwater intrusion

A classification of semienclosed basins is proposed in this chapter by following a tidally averaged momentum balance that compares drivers and modifiers of residual flow. Residual flow drivers are characterized by the nondimensional tidal Froude number, while balancing forces are typified by the Ekman number. Thus, the classification is contained in a parameteric space that considers estuaries, tidal rivers, vertically homogeneous lagoons, and frictionless tidal basins. The scheme can be regarded as the baroclinic tendencies shown by any basin, presented in the abscissa, versus the mixing tendencies, as represented in the ordinate. Also considered are situations in which wind stress competes with (a) density gradients and (b) tidal stress. This additional approach allows a dynamic description of basins where the dynamics are purely frictional, non-frictional (geostrophic), and Ekman-type. This classification includes semienclosed basins beyond estuaries.