The water main broke around 3 p.m. Wednesday afternoon at Dorsett Road and McKelvey Road in Maryland Heights.
While work crews were starting repairs on the water main, a gas line was ruptured. Laclede Gas workers have now been summoned to the scene to fix the gas line.
Water crews are having to wait till the gas line is repaired before they can start work on the broken water main.
In the meantime, traffic is being rerouted around the site.
Crews on scene say they hope to that has repairs finished before Thursday morning rush hour.