A large number of homes and businesses lost water on Wednesday due to a water main break near Browns Ferry Road and Cummings Highway.
Officials said the break was under a section of railroad tracks, making it difficult to reach.
It was expected to take some 12 hours to repair.
Water service was disrupted around 2 p.m.
At least one manufacturing plant sent workers home early.
Residents said it was the third Lookout Valley water main break since the first of November.