Aquaduct Ruins

Lots of good aqueduct ruin images and info, but Roman ones are worth a look
https://en.wikipedia.org/wiki/Roman_aqueduct

As my Chagrinspire game is post-industrial magic decopunk ruins, they are 4-8 times bigger with passages and have features, more like Victorian or later works like rail.

This might help too
https://elfmaidsandoctopi.blogspot.com/2024/01/celestial-cisterns-of-forgotten-evil.html

These indicate some ancient civilisation or greener times past. They are impressive megastructures you can mark in lines on a map, usually from where there was once water to a civilised area of water. They take a lot of work, so not on frontiers or near enemies. For heartlands to serve great cities.

Some systems might be a mix
The water can also carry cargo or aquaculture or rail lines or roads 
Common canals are just ditches and may be covered
Some sections act as bridges or you could use some of these features on bridges
to have tunnels through hills or mountains

Obviously, weird civilisations might use slime or blood or magic energy-conveying structures. You could have these structures in a vast cavern. 

d12 Condition
1-2 Broken pylons in rows
3-4 Broken pylons and some arches remain in sections
5-6 Intact sections, but damaged or stone and bricks robbed 
7-9 Mostly intact but worn or overgrown, needs work to operate
10-11 Old but serviceable condition
12 Imaculate via magic, possibly some automated repairs or still operational

d12 Structure Size
The biggest ancient ones got to 38m. Above 2m arcades of arches are used 
Assyrians went for wider, shallower open ones for barges; you could double the width for these
Broken sections might be only a few intact arches or long sections 2d6 x10m long
1-2 3m high x 1m wide with 1m duct
3-4 10m high x 1-2m wide with 1m duct  

5-6 20m high x 2-3m wide with 1m duct

7-8 30m high x 3-6m wide with 1.5m duct
9-10 40m high x 4-8m wide with 2m duct
10-11 50m high x 6-12m wide with  3m
12 100m high x 12-20m wide x 6m duct

d4 Vault of Duct
Ducts may be open and used or sealed
May have multiple ducts, especially multi-tiered
Made to be accessible for repairs and cleaning
1 Arch

2 Triangle

3 Trapezium

4 Rectangle

d12 Tiers
Each tier might have one or more ducts
Mid tiers might carry rail or road
1-8 One 
9-11 Two
12 Three

d12 Water Source
1-3 Lake
4-7 River
8-9 Swamp
10-11 Spring
12 Underground

d12 Canal Features
1 Crosses a river or water and may also be a bridge
2 Has a tunnel section through the hills
3 Dropshaft with water falling in steps to reduce pressure and structure damage 
4 Reservoir, underground cistern or sewerage system link
5 The water duct is open to the sky

6 Built for canal boats, possibly wider and shallower

7 Bird or bat colonies in archways, might be natural or to encourage game animals

8 Underground shafts or cisterns to manage water flow

9 Connected to local wells 

10 Distribution basins where water exits the aqueduct to serve the local area

11 Setting a basin where rubbish and impurities are removed frequently

12 Stalactites hang from archways

d12 Various Current Structure Uses
Apply a d3 of these on a segment of the aqueduct
You might have past uses no longer operational but visible
1 Drinking Water - provides local drinking water or a distant area
2 Industry - supplies factories, mines, dyers, tanners and other workers
3 Farms - supplies local agriculture or a distant farm area 
4 Fish Farms - eel, fish and lampreys might be farmed in the water
5 Water Transport - moving of goods in barges or boats
6 Road - might include road or tracks or be used as a bridge or include a walking trail
7 Defence - may include watch stations, outposts or watch towers 
8 Communications - signal towers or a postal service, or messenger stations
9 Shacks - possibly homes of farmers, poor or homeless, might live inside the structure
10 Structure - government buildings, coach inns, taverns, shops, tombs, storage or a mill
11 Overgrown - covered in trees, vines, moss, lichen, possibly produces food or game
12 Dungeon - The interior pylon structure has crawlspaces or larger ones, but may also include other structure ruins, entrances to caverns or underground tunnels. Huge structures easily hold dungeons inside, which might be occupied by pesky humanoids or demihumans

d12 What Lives Inside Crawlspaces
1 Rat swarm or giant rats

2 Giant frog or slamander

3 Giant beetles or isopods 

4 Bat swarm or giant bats

5 Worm swarm or giant worms

6 Giant spiders and webs

7 Humans from crazed loners, cults, a gang or some hidden outcasts

8 Goblinoids or kobolds

9 Species of beast folk might be small type or bigger if room

10 Changelings or lycanthropes

11 Undead, corporeal or spirits, some types hate running water

12 Spirit may appear human or animal, or might be an insubstantial type

d12 Unusual Aquaduct Ruin Sections
1 Secret residents inside the pylon structure, and may even have a garden or house on top
2 A species of vermin has infested the pylon structure and is bothering locals

3 Bandits use as a secret hideout 

4 Humanoids have occupied secretly at first

5 A criminal guild uses it as a hideout and safehouse for the gang

6 Cult uses as a secret shrine and meeting place

7 Repurposed as a tomb at one period

8 Turned into a business like a tavern, barn or trading post using a structure

9 Trapped cache or treasure of a faction, highly vindictive if robbed
10 Fortified and used as a watch tower by a faction or the crown

11 Overgrown with vegetation and life

12 Local monster lair or nest, possibly a flying one
13 An ancient shrine used by an active church
14 Shacks built into the structure, some off the ground for safety

15 Used as a prison with gallows and gibbets, and cramped cages hanging

16 Witch or holy hermit lives on top of the pylon and might talk to strangers

17 Gangs come here to get high, smash stuff, write offensive slogans and fight

18 Used as a dump for rubble, farm waste and rubbish dredged from rivers and canals

19 Scavengers are stripping for building stone or bricks from a quarry by locals

20 large trees and vegetation on top, and no visible way up, a good hiding place for a bird,  climber

d12 Other Stuff Nearby...
Maybe consider the water table. Has it raised or lowered? Possibly salty water or tainted?
People in the modern era often see these ancient wonders on certain roads, and they may be a popular tourist trip for the rich and pilgrims to see them and follow them in the countryside. 
1 Major roadway 

2 Well

3 Cistern entrance

4 Farm or herders

5 Air shafts for underground water tunnels

6 Ruined houses, possibly a settlement 

7 Ruined villa

8 Ruined tower

9 Ruined fort or castle

10 Ruined graveyard

11 Grove of thick trees planted by the ancients thrives

12 Prehistoric ancient stone monoliths, even the ancients thought sacred

To do
d100 rural bridges
wizard licenses