When is a lock not a lock?

This is the Teddington Locks, where the Thames finally stops being influenced by the tide.

But further downstream there is this - and if it is not a lock, then what is it?

It is Richmond Lock, which is technically a half-tide lock and barrage. You can sail straight through at high tide and about two hours either side. Its there to keep the waters between here and Teddington a depth of 1.72 m or more.

It is the only lock managed by the Port of London Authority (PLA), the others being the responsibility of the Environmental Agency.

The point on the river where the remit of one ends and the other begins is marked by this obelisk, which many on the tow path no doubt go passed without noticing.

I did - which was another reason for heading back this weekend!