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.