Tidal rivers are freshwater regions of tidal influence. That means that water level rises and falls daily with the tides but oceanic salinity is not present. This makes tidal rivers distinct from “regular” rivers, which aren’t affected by tides, and also from estuaries, where freshwater and salt water meet.
Until recently, there has been relatively little research carried out in tidal rivers because they’re a bit of a no man's land: they aren’t exactly rivers, and they aren’t exactly estuaries, either. Today, there is increased interest in understanding tidal rivers’ role as a missing link between fluvial terrestrial and estuarine environments.
Above, an aerial view of the confluence of the Amazon (brown color) and Tapajós (black color) tidal rivers, near the upstream limit of tides at Santarém, Brazil.