The problem: how to ensure a regular supply of water using gravitation only? In most of the cases water was transported underground. However, at some places that was not possible. That's why the Romans built bridges/aqueducts of stone and concrete. Such a conduit had to have a precise angle so the water could flow evenly.
 

For most of their length the early aqueducts were simply channels bored through the rock, from the water intake in the hills almost to the distribution cistern (i.e. in Rome). The depth of the channel below ground varied so as to maintain a constant, very shallow gradient throughout the length of the aqueduct; sometimes it even went through hills or mountains. Vertical shafts were then bored at intervals to provide ventilation and access (for maintenance purposes). Only in the final stretches was the conduit raised on arches, to give a sufficient head for distribution of the water within the city.

In order to keep the gradient constant, the aqueducts took a roundabout route, following the contours of the land (see the picture of the Gard aqueduct below). As time went by, Roman engineers became more daring in the construction of high arches to support the conduits across valleys and plains and some of the later aqueducts were as much as 27 meters above ground level in places. More details can be found at