Coedty Reservoir is a reservoir in Snowdonia, North Wales. It is fed by the waters of Afon Porth-llwyd which flows from Llyn Eigiau.
In 1925 the Eigiau dam broke, and the flood of water also burst Coedty dam. The resultant flood caused the loss of 17 lives in Dolgarrog.
The dam, originally built in 1924, was rebuilt in 1926, and altered again in 1956.
Coedty dam is a balance tip dam, water flowing from it as the water reaches high levels.
There is a spectacular gorge below Coedty Reservoir, reached from Dolgarrog and used by outdoor pursuit groups. Gorge walking here, however, can be dangerous because of the sudden releases of water from the reservoir which can occur.
Water from the reservoir is also carried via a large black pipe down to the aluminium factory in Dolgarrog.
The eastern side of the lake is skirted by the trackbed of the former Eigiau Tramway, created to aid the construction and maintenance of Llyn Eigiau dam. Nearby the Cowlyd Tramway branched off, built to aid the construction of the dam at Llyn Cowlyd.
The reservoir lies at a height of 900 ft, and measures some 12 acres in size. It contains brown trout.