The aqueducts were largely a gravity system. They had to
keep the water at a certain level because if they lost that level, it was hard
to get it back up again. So to maintain a slope towards the city and to bring
it in at a high enough level, you had to keep the channel at a certain height.
So when that channel came to a dip in the landscape they built an arcade or a bridge
to take the water over it. Look around this site as well. It provides tons of information.