The Weir That Won't Hold

A rain-swollen millpond is pressing against a timber weir whose gate has been jammed by flood debris. The structure is failing, not exploding: each round brings another groan, a stronger surge through the spillway, and less safe ground along its banks. Characters can try several practical solutions, protect people from the water, or accept damage to the market edge while they work. The hazard is resolved when the weir is stabilized or the gate gives way and the pond drains through the breach.

Moderate; use initiative while the weir is actively failing. · Environmental / waterworks failure · Level Suitable for two 4th- or 5th-level adventurers.

The Weir That Won't Hold

A rain-swollen millpond is pressing against a timber weir whose gate has been jammed by flood debris.

Disable: 12

At a glance

Level

Suitable for two 4th- or 5th-level adventurers.

Moderate; use initiative while the weir is actively failing.

Disable During initiative

Initiative The hazard acts on initiative count 20

The hazard

Environmental
Water
Bludgeoning

Trigger: The hazard triggers when the characters approach the failing weir or enter the spillway's affected area. If they are already present when play begins, it triggers on initiative count 20 of the first round.
Disable: During initiative, the characters need three stabilization successes to secure the weir. A character can use an action for a plausible intervention, such as relieving pressure, clearing the obstruction, or bracing the gate. The listed countermeasures give sample checks and methods, not exclusive solutions. Set a DC of 14 for a risky but practical action, or when two characters coordinate and both contribute an action. Award a success for a sound approach that reasonably controls the water, even if it uses a different ability or equipment. On the third success, the flow drops to a manageable level and the hazard ends.
Initiative: The hazard acts on initiative count 20, losing ties. Roll initiative for the characters normally. On its first turn, it triggers; the surge repeats at the start of each subsequent hazard turn until the weir is stabilized or breaches.
Routine: On initiative count 20, each round, resolve the Surging Spillway effect for creatures in the affected area. Then the characters can take their turns and attempt countermeasures. Once the characters earn three stabilization successes, stop the routine. If they do not stabilize the weir, it breaches at the end of the third hazard turn; resolve Gate Gives Way and stop the routine.
Reset: The hazard does not reset once stabilized or breached. It requires repair and debris removal before the weir can function normally again.

Effects

Surging Spillway
DC 13
When the hazard triggers and at the start of each subsequent round before it is stabilized or breaches, creatures in the spillway or within 10 feet downstream of the weir must make a Dexterity saving throw. On a failed save, a creature takes bludgeoning damage, is pushed 10 feet downstream, and falls prone. On a successful save, it takes half damage and is not pushed or knocked prone. A creature pushed into the deeper channel can stand and move normally, but the water is difficult terrain for creatures without a swimming speed.
Gate Gives Way
DC 13
When the weir breaches, creatures in the spillway or within 10 feet downstream make one final Dexterity saving throw against the Surging Spillway effect. The resulting rush carries loose debris into the market edge, damaging goods and structures there, but the water then runs down and the hazard ends. The surge is not intended to trap characters underwater or cause instant death.

Special rules

Work in the Surge.The hazard's initiative count represents the growing pressure, not a creature taking actions. Characters can work on the weir while it surges. A character in the affected area can still use an action to attempt a countermeasure, but must also make the Dexterity saving throw when the surge occurs.

Teamwork.A character who spends an action helping another character with a countermeasure grants advantage on that character's check if the help is practical and both can reach the work. Apply the normal rules for the Help action.

Evacuation.A character can use an action to move people from the muddy banks to higher ground. Those people are safe from the breach surge, but evacuation does not earn a stabilization success. If the weir breaches, the market edge takes damage and loose goods are swept away.

Stop it

Signs of dangerA rising waterline has covered the lower stones along the pond. · The timber gate shudders, and its braces bend with each surge. · Branches, reeds, and muddy clumps are wedged against the gate. · The spillway's brown water is already fast enough to carry loose boards downstream.

Countermeasures

DC 14Relieve pressure by opening a sound sluice or cutting a safe overflow channel. A character can make a Strength (Athletics) check to move a jammed board or gate; suitable tools and a sound plan may justify another ability check at the DM's discretion.
DC 14Free the obstruction. A character can use an action to make a Strength (Athletics) check to haul branches clear, or attack the obstruction (AC 13, 18 hit points). Each successful check or destruction of the obstruction earns one stabilization success.
DC 14Brace the crooked gate with a stout beam, a suitable shield of timber, or a rope-and-team effort. A character can use an action to make a Strength (Athletics) check. Two characters working together can instead each use an action and succeed on a Strength (Athletics) check; together, their effort earns one stabilization success.
DC 14Use ropes and teamwork to hold a gate or haul debris from a safer position. Award one stabilization success for a successful Strength (Athletics) check, or for a comparably risky and well-described plan using another ability.
Evacuate people from the muddy banks and nearby market edge. Each character can use an action to move a small group to higher ground. This prevents them from being caught in the eventual surge, but does not count toward stabilization.

In short

Stabilize the weir with any three successful actions that relieve pressure, clear the obstruction, brace the gate, or otherwise control the flow.

Its story

What it looks like

Rainwater boils over the weir in brown sheets.

Why it is here

The weir diverts ordinary flow from the millpond; the hazard results from flood pressure and a jammed gate, not a deliberate trap.

Who made it

Unknown; ordinary millwrights and workers built and maintain the timber works.

Lore

A timber weir holds back the millpond and channels excess water into the spillway.

Where it is found

Rain-swollen millpond, timber weir, spillway, and muddy banks near a medieval market town.

The Weir That Won't Hold

The Weir That Won't Hold

The hazard

Effects

Surging Spillway

Gate Gives Way

Special rules

Spotting it

Discovery

Perception

Stop it

In short

Its story

What it looks like

Why it is here

Who made it

Lore

Where it is found

Media

Turn The Weir That Won't Hold into a sheet