Hundreds of villagers have been given the all-clear to return to their homes after earlier being advised to leave Pennal, North Wales, when cracks were detected in a nearby dam.
Superintendent Andy Jenks-Gilbert said the evacuation was a precautionary measure.
A crack had been found in a wall and a small amount of water had escaped.
"Should the wall be completely breached, the contents of the reservoir will go into the river which flows through the village of Pennal.
"We have managed to avert a very serious situation," Gwyn Jones of North Wales Fire and Rescue Service told the BBC.
"The pressure on the reservoir was immense, so public safety was paramount.
"Over the course of the day, we've been able to undertake two releases of water from the reservoir. We were closely monitoring the level of the river, and everything was satisfactory after those two releases.
"Based on that information and survey from specialist pump operators on the scene, we were able to give the advice through North Wales Police that residents were able to return to their homes."
Pennal resident Andrew Dickinson had earlier told ITV News that police "suggested that we evacuated, not forced."
A police spokeswoman said residents had been transported to Machynlleth Leisure Centre.
"Staff from the local councils, representatives from the local water board, North Wales Fire and Rescue Service and officers from North Wales Police are at the scene working to stabilise the situation," she said.
A crack had been found in a stone wall at the reservoir and a small amount of water had escaped, he said.
"Should the wall be completely breached, the contents of the reservoir will go into the river which flows through the village of Pennal, which is around half a mile from the reservoir."