A fault has been detected at a flood storage reservoir in the Vale of Glamorgan, Natural Resources Wales (NRW) said.
As part of a regular inspection of flood defences on the River Thaw, Cowbridge, a "minor fault" was found.
NRW said it would be repaired over the next few days and added the "structural integrity of the reservoir" was sound.
In a statement they reassured residents they were "doing everything possible" to keep them safe from flooding.
"To carry out the repairs we will be drawing down water from the reservoir in a controlled manner which will result in the levels of the River Thaw visibly going up and down over the next few days," the statement added.
A flood alert is currently in place for the river and officers are continually monitoring the levels at the reservoir and downstream.