Cobble stones used to shore reservoir
Cobble stones that once lined the streets of Bangor and Caernarfon in Gwynedd have been used to repair a reservoir in the Gwydr Forest near Capel Curig in Conwy.
A new spillway was also designed after concerns the dam would not be able to cope in extreme weather.
The Llyn Goddionduon reservoir supplies water to the army training camp at Capel Curig.
The reservoir lies in a Special Area of Conservation.
"Soldiers and cadets from all over Britain and Germany come to take part in exercises over the mountains, forests and rivers of Snowdonia," said Gavin Queen, Forestry Commission Wales country civil engineer.
Potential problems at the site were discovered after a survey by the Environment agency, following changes to reservoir safety legislation.
It raised concerns the dam and spillway might not be able to cope with the volume of water in the event of extreme weather.
Forestry Commission Wales and the Ministry of Defence have joint responsibility for the site.
The spillway was redesigned by Forestry Commission Wales civil engineers Peter Oxbury and Rory O'Donnell to make it safe, whilst retaining its "sensitive characteristics".
They built a new dam using a stone-built basket called a gabion, sealed with a layer of clay, beneath a spillway made from the recycled 'setts' or cobbles.
"The stones were granite cobbles which were originally used to pave the streets of Bangor and Caernarfon before they were replaced by tarmac," said Mr Queen.
He said the work had been a challenge.
"The sensitive nature of the site meant the water level had to be maintained," he added.
As well as making the site safe, engineers built an access track, enabling the public access to the reservoir.