River Thames

[Pharlap]

Where does the salt water join the fresh water. With the drought, how far does the fresh water travel from its source now and how far should it travel normally.

[Catso]

The fresh/salt interface is gradual, large, and it moves in and out of the estuary/river with the tide.

I've read that on high tides, the river in the city tastes slightly salty.

I would doubt that even at low tide the fresh water gets more than a mile or two out into what could be called the sea.

Whereas for the Amazon, it gets tens of miles out to sea. I've read you can get fresh(-ish) water even out of sight of land.

[JoolsTwo]

Fresh water and salt water meet at Teddington Lock.