Closed pipes were occasionally used to cross valleys by the "inverted syphon" method; the pressure forced the water down and up again on the other side, to a level slightly lower than before. This system however was costly, as it required lead pipes (lead had to be imported from Spain or Great Britain) and it was difficult to make joints strong enough to withstand the pressure; so arches were far more common.

 Where the aqueduct went through impermeable rock it was not lined, but where the rock was porous, and where the conduit ran on arches, a layer of impermeable concrete could be applied to form a waterproof lining (opus signinum).

Every now and then there was a sedimentation tank, where the flow of water slowed down and impurities were deposited.