Peterborough Lift-lock seen from the south side - built in 1904
U is for UP, and the top of the lift-lock is certainly UP - 65 feet in the air. Boats that wish to travel westerly on the Trent-Severn Waterway must be raised that much to continue. See the large pan on the left side of the lift-lock? It's raised and at the top. A similar pan is on the right side mostly under water in this photo.
A hydraulic lift raises the pan with boats in it to the top.
Top side of the lift-lock
Another photo of the top side
I am linking to Jenny Matlock at Alphabe-Thursday HERE