Aqueducts moved water through gravity alone, along a slight overall downward gradient within conduits of stone, brick, or concrete; the steeper the gradient, the faster the flow. Most conduits were buried beneath the ground and followed the contours of the terrain; obstructing peaks were circumvented or, less often, tunneled through. Where valleys or lowlands intervened, the conduit was carried on bridgework, or its contents fed into high-pressure lead, ceramic, or stone pipes and siphoned across. Most aqueduct systems included sedimentation tanks, which helped reduce any water-borne debris. Sluices and castella aquae (distribution tanks) regulated the supply to individual destinations. In cities and towns, the run-off water from aqueducts scoured the drains and sewers.

Answer this question: what force did the romans use to move water? Let's give stream of consciousness first:
This is the relevant information: Aqueducts moved water through gravity alone, along a slight overall downward gradient within conduits of stone, brick, or concrete; the steeper the gradient, the faster the flow.
So, the final answer is gravity.