Martian Aqueduct System

Aqueducts were an important aspect of Martian terraforming and one of the lasting monuments of the original Martian colony. As the name implies, Aqueducts are used to convey water across Mars, especially through terrain gradients that would not otherwise support water flow such as isolated canyons. The Aqueducts of Mars consist of a series of complex underground canals and tunnels that culminate in thousands of kilometers of artificial waterways.
Transporting water from the polar ice caps and underground reservoir at Utopia Planitia to more useful regions was one of the main reasons for constructing the Aqueduct System. Without these Aqueducts, large bodies of water like the Hellas Sea and Mariner Sea would have taken centuries to develop through rain and snow precipitation, if at all.
Aqueducts range in size from massive main-trunk canals with a megaliter per second capacity, to smaller tubes and tunnels that act as tributary rivers or bridges between lakes at different altitudes. Most Aqueducts have multiple channels to handle seasonal flows of different heights.
Construction of the Aqueducts was largely automated, based on complex water flow simulations run at various stages of terraforming. Boring and tunneling drones developed the water passages, while construction drones laid down locally-fabricated concrete blocks to make the tunnels more resilient to erosion.
The Aqueducts were designed to be self-sufficient with minimal upkeep, and as a result most remain functional hundreds of years later. Only a few have suffered collapses and cave ins, and usually due to outside circumstances like unforeseen cavern/tunnel intrusion, human conflict, or unanticipated excess water flow. Some of the Aqueducts that were once essential to terraforming have fallen into disuse. These are sometimes converted into subsurface habitats.
The Martian Aqueduct System around Valles Marineris is the most complex, and many tributary canals haven't been fully mapped after recolonization. The Native Martians use the Aqueducts, along with naturally occurring tunnels, as hidden subsurface passages and habitats. The Aqueducts present a particularly dangerous path during wet seasons when flash floods can rapidly fill an empty aqueduct without warning, sweeping away anyone unfortunate enough to be caught inside.