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 technology.
By using building blocks it became easier to construct big
solid arches. These had to carry the upper, water level. Now
uneven terrain could be crossed and the length of the
aqueduct shortened considerably.