It was recently discovered by a routine Network Rail inspection that fast-flowing water from the River Tame has caused the riverbed to wash away.
Approximately 2,500 tonnes of rock armour, which are large boulders, is being installed at the foundations of the structure.
The immediate repairs will make sure the viaduct's foundations are not at risk of erosion and trains can safely travel on the Hope Valley line for years to come.
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Chris Pye, North West infrastructure director at Network Rail, said: "We're carrying out vital work to protect Reddish Vale viaduct and the riverbed from erosion caused by fast-flowing water to ensure passenger and freight trains can continue to run safely on the Hope Valley line.
"I would like to thank local residents and businesses for their patience and understanding while the work takes place. I appreciate it’s not ideal to have large machines near your homes. We're doing all we can to limit the impact wherever possible."
Train services are still running over the structure at the usual 50mph line speed.
Chris Jackson, regional director at Northern said: "Improvements like these are essential in delivering our long-term strategy to create a better railway for the north and is a great example of the rail industry working together to improve the network.
"We are sorry for any disruption while the work takes place and our customers can be assured that both Northern and Network Rail will do everything possible to keep them on the move."
