The tide reigning before a coast spreads upwards in rivers with identical periodicity, but the dissipation of the energy gradually reduces the amplitude and the tide ends up being imperceptible, with this being the boundary of the maritime part of the river (this boundary is of over 1000 km for the Amazon River).
In an estuary and even more so in a river, the high tide is of much shorter duration than the low tide; it may even be that the ascent of the water up-river is almost instantaneous, this is the moment of the tidal bore, breakwaters that enclose the whole bed of the river which in turn also surges quickly upwards.
Pororoca. Amazon River, Brazil
It occurs in the Amazon River in spring tides, and waves of up to 4 m in height are formed which are taken advantage of by surfers from around the world who come to the mouth of this river to surf tens of kilometres up-river with the waves. Despite its devastating effects on the banks of the river, it is a phenomenon much appreciated by fishermen as a great number of both fluvial and marine species gather to feed and reproduce and therefore fishing at these times is both most attractive and emotional.