How do rivers (such as the Thames in London) have tides?

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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 whole river isn’t tidal, but the estuary is. Estuaries are where the fresh water meets the sea. The tides in the river are caused by the sea, not by the river itself.

The constant supply of a river is the bay, sea, etc. When that bay experiences high tide, there is more water in the bay. That water wants to go somewhere, so it proceeds downriver, and increase the water level in the river as well. The smaller and shallower the river, the greater tide it will experience. Eventually, the extra tidal water reaches a point a larger part of the river, where it has covered a certain area of river, and thus the extra water flow has less of an effect.

London is only ~30′ above sea level and the tides can vary about 25′.
You have the force of the outflow of the river (or outgoing tide) vs the inflow of the water coming in from the tide. In some places (Turnagain arm in Alaska) this will actually cause a wave where the two meet, called a tidal bore.

You are correct in that there is a constant flow of water down the river. However that water eventually meets the sea, and in ELI5 terms: there is a lot more water in the sea than in the river!

So when the sea level rises due to the tide, it literally pushes the water back up the river and the river’s level rises too.

As others have mentioned, in some rivers the geology allows you to actually see the water being forced back upstream in the form of a tidal bore. In the UK there is sometimes a strong tidal bore on the Severn — strong enough to surf on! — and I’ve seen it myself on the Mersey where it narrows around Runcorn. Also if you go to Conwy in Wales, you can stand on the bridge by the castle and watch the river flowing inland away from the sea when the tide is coming in. I’m sure there are many other examples in the UK and abroad.

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.