Lancashire Police says a large number of stones from the walls between Rivington and Anglezarke have been stolen and are appealing for information.
Some of the walls are more than 200 years old, dating from before the reservoir was built.
They play a vital part in keeping the embankments back and preventing vehicles from leaving the carriageway in an accident.
Officers added that the offenders will have taken a “significant amount of time” to move the stones due to their weight and the number taken.
Anyone with information which could help the police should email 620@lancashire.police.uk.
