Roman aquaducts are famous, but didn't the water freeze during winter? Where did the cities get their water during winter?
EDIT: running water doesn't freeze as easily as motionless water, which leads to an important bit of info we need: how fast did the water run in Roman aquaducts? We might also need to know volumetric flow rate.
EDIT 2: Of course rivers don't freeze solid. I know it's only a surface or skin of ice. However, once the surface freezes, how did they access the liquid water underneath it? How did they pump that into their aquaducts?