The River Towy (Welsh: Afon Tywi) is the longest river which entirely runs through Wales.
It rises on lower slopes of Crug Gynan in the Cambrian Mountains grid reference SN802631 and flows through the Tywi Forest forming the border between Ceredigion and Powys.
The Llyn Brianne reservoir was formed here in 1972 by damming a section of the river in order to store winter rain so that it could be released into the river during dry periods. The purpose behind this construction was to support the new abstraction at Nant Garedig that was destined to supply a large swathe of south Wales with drinking water. The flow in the River Tywi would have been unable to sustain such an abstraction were it not for the release of water from the upland reservoir.