The River Thames in London is a tidal river, it’s not the only one in the world, but it’s the only one I know of. How does a river which surely has a constant supply of water flowing into it have a tide?
In: Earth Science
The shortest answer is: a river can be almost (but not quite) “flat” for up to several miles before it drains into the sea.
On a beach you are familiar with the tide rising and falling, sometimes over very long horizontal distances (a mile or more in some cases, if the beach is relatively flat). A tidal river is no different, except that the area being inundated by the tide happens to have a river draining into it. Instead of covering a mile or three of beach, the tide rushes up and down the river channel that same distance. It is NOT the entire river that sees a tide, only the parts that are at or below the level to which a tide can rise. In London the Thames is quite flat (relatively speaking) and nearly at sea level for several miles upstream from its endpoint, meaning the tide can reach up to whatever that point is “upstream” instead of “upbeach”. It’s a normal tide, there just happens to be the Thames in the way instead of a seaside cliff, so the tide goes further inland to reach the same elevation.
Instead of hitting the cliff ten feet up (or thirty feet up, or whatever) the tide rushes up the river until it is thirty feet above sea level, even if that thirty feet (or ten feet, or whatever) is miles upstream.
Hopefully that helps.
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