When tides enter estuaries or basins, the boundary conditions change as the geometry changes drastically. The water depth becomes shallower and the width decreases, next to that the depth and width become significantly variable over the length and width of the estuary or basin. As a result the tidal wave deforms which affects the tidal amplitude, phase speed and the relative phase between tidal velocity and elevation. The deformation of the tide is largely controlled by the competition between bottom friction and channel convergence. Channel convergence increases the tidal amplitude and phase speed as the energy of the tidal wave is traveling through a smaller area while bottom friction decrease the amplitude through energy loss. The modification of the tide leads to the creation of overtides (e.g.  tidal constituents) or higher harmonics. These overtides are multiples, sums or differences of the astronomical tidal constituents and as a result the tidal wave can become asymmetric. A tidal asymmetry is a difference between the duration of the rise and the fall of the tidal water elevation and this can manifest itself as a difference in flood/ebb tidal currents. The tidal asymmetry and the resulting currents are important for the sediment transport and turbidity in estuaries and tidal basins. Each estuary and basin has its own distinct geometry and these can be subdivided in several groups of similar geometries with its own tidal dynamics.