that Temple is deep, just wait until you get to a "proper" lock, Temple and Hurley are about the shallowest (go on everybody and rush to the depth tables....), they are tiddlers! Have a look at Marlow, which also has nasty upstream sluices.

Not meaning to be disingenuous as even three feet can be frightening on a first visit.. You were unlucky to use one of the remaining unconverted locks. Your problem relates to the leave lock empty "rule" which nobody really understands and that includes the keepers. I suspect that it may be to discourage the growth of algae?

Indeed the rule appears to have been withdrawn for the later electrified locks.

A definitive, logical explanation is long overdue....