How does the Thames flow out when the Barrier is closed?

Question

wouldn't closing the Thames Barrier make the upstream river burst out of its banks?

Answer 1

IIRC The Thames is a classic 'drowned river`.
When the tide rises, the tide comes in faster than the river flows out.
The barrier allows fresh water to build up when the tide is rising and keeps the salt water down stream.
The water level doesn't exceed the 'high tide level
without the barrier in place so flooding isn't an issue.

Answer 2

Makes it rise. One principle is that you can never stop the flow of water, you can only redirect it.