The Split Aqueduct

RoomTrap RoomHazard4 exits

floodedmechanismtrap-disarmdrowning

Illustration: Room 24 — The Split Aqueduct

The corridor ends at a channel of fast black water eight feet across, running left to right with the sound of a mill-race. Somebody built this: the channel is dressed stone, the lip is worn smooth, and three iron wheels stand on the near bank on stub pillars, each with a bull's head, a wave, or a fish beaten into its hub.

Downstream, where the water goes into the wall, there is a grille. Something long is lying against it, and the current is holding it there, and it has not moved since you came in.

DM summary. A working aqueduct with a three-wheel sluice gallery, a lethal grille at the downstream end, and an eel the size of a man that the room treats as weather rather than as a monster. The party's job is to be on the far bank with everything they arrived with.

Terrain & Features

Battle-map thumbnail: The Junction Chamber

Battle map: The Junction Chamber — page 39, 55 × 75 ft (one half of The Sewers)

The junction chamber's straight canal is the aqueduct channel: run it across the room's middle, chalk the three crossing stones in it, and draw the iron grille where it leaves the wall. The walkways are the two banks.

Dimensions. 50 ft × 35 ft, ceiling 12 ft. The channel runs the full 50 ft east to west across the middle, splitting the room into a near bank (10 ft deep) and a far bank (15 ft deep, with the exits).
Light. Dim, from four wall-niches of luminous lichen. The water is black and nothing can be seen in it from the bank; Perception DC 16 from directly above, with a light source held low, makes out the shape moving in it.
The channel. 8 ft across, 6 ft deep, current 30 ft per round. A creature that enters it is swept 30 ft downstream at the end of each of its turns unless it makes an Athletics DC 15 swim check that round; a creature that is prone, grappled, or carrying more than half its capacity has disadvantage.
The grille. Where the channel leaves the room, an iron grille of 6-in bars. Anything swept into it is pinned by the current: 4d10 bludgeoning on arrival, and each round pinned is one Constitution save DC 14 against suffocation. Getting a pinned creature off the grille is Athletics DC 16 from the bank with a rope, or DC 18 unaided by the pinned creature itself.
The crossing stones. Three cut blocks stand in the channel at 12, 25, and 38 ft along, their tops 4 in under the surface at present depth. They are the reason the sluice puzzle exists.
The wheels. Three iron wheels on the near bank, 6 ft apart, marked Bull, Wave, and Fish. Each is stiff: Athletics DC 12 to turn one a full revolution, one revolution per action.
The eel. A blind aqueduct eel, ten feet of it, lying against the grille because that is where the current puts food. It is a hazard, not a creature: it has no stat block, rolls no initiative, and cannot be fought. It bites what enters its water and it does not leave the channel.

Traps / Puzzles / Checks

The sluice gallery. Upstream of this room the aqueduct splits three ways. The wheels close those three gates. The pattern is legible from the bank because the water tells you: each closed gate drops the level in the channel.

Wheels closed	Channel depth	What that means
None	6 ft, fast	Crossing stones submerged. Swim only
One	4 ft	Stones still under. Swim, current 20 ft
Two	Ankle-deep	The stones are dry stepping stones. This is the solved state
Three	Dry	The header tank overtops upstream and the whole thing comes back in a wall: everyone in the channel is swept to the grille, no save

Element	Value
Spot the marks	Investigation DC 13 on the wheel hubs — the Bull hub is polished and the other two are not, because the Bull gate is worked daily
Understand the tank	Investigation DC 14 at the channel lip, reading the tide-marks on the stone: three lines, and the top one is above head height
Refill cadence	Three rounds. Two gates closed, the channel runs shallow for exactly three rounds, then the header tank spills over and the level comes back at the top of the fourth
Hold it open	A spear-shaft, crowbar, or javelin through the wheel spokes (no check) holds one gate; the second must be held by hand, one character on the wheel
Failure state	Anyone still in the channel bed when the level returns is swept: Athletics DC 15 to reach a bank, or 30 ft downstream toward the grille
The eel	Anything in the water at any depth: Dexterity save DC 14 on entering the channel and at the start of each turn in it, or take 2d10 piercing and be dragged 10 ft downstream

Telegraph, always. One round before the tank spills, the wheels shudder and the water pulls back off the stones with a sucking noise. Describe it. Nobody should die in this room without having been shown the round before that it was coming.

The eel and the grille together are the only lethal combination here, and both are avoidable. A party that crosses in three rounds on dry stone never meets either.

Exits

AObviousThe far-bank arch, directly opposite the way you came, wide and dry. No check — once you are on the far bank. Note it aloud early: the room gates crossing, not leaving.

BObviousThe near-bank service stair up to the sluice gallery, and out along the aqueduct's maintenance walk. No check.Rider: the walk runs above the water for a hundred yards and comes down elsewhere — the party arrives dry but has been in the open, and anything that watches the channels has seen them.

CConditionalThe grille itself, which lifts on a chain winch once the channel is shallow (Athletics DC 15, two characters). Beyond it the water runs down a pipe a person can crawl.Rider: you arrive soaked to the skin — fire damage against you is halved for one minute, and every check to hide, climb, or handle rope in the next room is at disadvantage until you dry off.

DHiddenA silt-choked overflow mouth in the far bank's downstream corner, invisible until the level drops. Investigation DC 14 while the channel is shallow.Rider: you come out plastered in aqueduct silt and smelling of nothing but wet stone — for one room, nothing can track the party by scent, and neither can they follow anything by its trail.

Loot

None. The channel has been washing this room out for centuries; anything droppable is a mile downstream and everything else is bolted to the floor. The eel has eaten what it has eaten and the party is welcome to go and look.

Scaling — APL 6 / 12 / 15

APL	Adjustment
6	Two wheels, not three, and closing one is enough to drop the level. Grille damage 2d10, eel damage 1d10. All DCs −2. Shallow window four rounds
9	As written
12	The Fish wheel is seized (Athletics DC 18 or Thieves' Tools DC 16 to free it) and must be freed before it can be turned. Grille 10d10, eel 4d10. DCs +1. Shallow window two rounds
15	As APL 12, plus the header tank spills on a two-round cycle whatever the party does, so the crossing is a rhythm rather than a window; and a second eel lies under the near bank. DCs +3, grille 10d10

DM Notes

Pacing: ten minutes. It is a crossing, not a set-piece, and it should move at the speed of the water. Behind schedule? Start it with one gate already closed and a crowbar already through the spokes — the party turns one wheel and walks over. Four minutes.
Run the three-round window on the table, out loud. Count it down: "three — two — the water pulls back off the stones — one." Hurrying a party across stepping stones with a real number ticking is worth more than any amount of described menace.
The eel is not a fight and you should refuse to make it one. If a player attacks it, it goes under and the water is exactly as dangerous as it was. This is deliberate: the deck needs at least one room where the monster is scenery and the mechanism is the enemy.
Table safety: drowning. Pinning against the grille is the sharpest image in this batch of rooms. If drowning is on your table's veil list, run the grille as a snag that costs gear and a round of panic rather than air, and drop the Constitution saves entirely; the room still works.
Clue drip. Two objects: the salt crust and the sea-wrack tangled on the upstream side of the grille, which means this water reaches the sea (X16, and X11 more strongly), and the surveyor's chalk mark on the sluice gallery wall — a crew number and an arrow — in the same hand as the marks in other rooms (X14).
No warband here. If you need a wandering encounter after this room, the party is wet and loud; use whatever the previous room fielded, at half strength, and let the noise of the wheels be the reason it found them.