West Yorkshire Fire and Rescue Service said high water levels were putting a dangerous amount of pressure on the structure.
Crews from Halifax and Todmorden are at the scene.
Six homes have been evacuated and Lumbutts Road is closed.
Calderdale Council warned people to avoid the area.
The council posted on Twitter: "Lumbutts Road in the #Todmorden area is now closed, and will remain closed until at least midnight tonight. This is due to a reservoir issue, which is being managed by the emergency planning team and West Yorkshire Fire and Rescue Service.
"If you are travelling tonight please avoid the area. #safercalderdale"
It comes after 1,500 homes were evacuated in the Derbyshire town of Whaley Bridge after flooding caused damage to a dam at the nearby Toddbrook Reservoir.
Derbyshire Fire Brigade has said the dam is at 'real risk of collapse'.