nope its not a dam, my understanding is a dam is a permanent structure which restricts the water flow.
The Thames Barrier, is made up of many kinda bowl /dish shaped things which normally lie flat but when the water levels need to be controlled, they sort of swing up to form a barrier,
Thats all i remember from my visit there in prep school.
It consists of a number of movable half round wedges that normally sit clear on the barrier base and swing up to block the individual channel. Been through it a few times.
It is a form of removable dam which, at times of the authorities choosing, is intended to hold back water under certain circumstances. It normally sits below water level but can be raised to close from bottom to above water level - a bit like a gate which can pen in/out animals or traffic, or allow them through, hence the name "barrier". It is the dam equivalent of a swing bridge.