The River Towy (, ) is one of the longest rivers flowing entirely within Wales. Its total length is . It is noted for its sea trout and salmon fishing. Route The Towy rises within of the source of the River Teifi on the lower slopes of Crug Gynan in the Cambrian Mountains. Flowing through the steep hills of the Tywi Forest, it forms the boundary between Ceredigion and Powys. The river flows generally south-westwards through Carmarthenshire, passing through the towns of Llandovery and Llandeilo. Its total length is . with numerous tributaries that include the Cothi, Gwili, Brân, and Doethie. In Carmarthen, it is joined by a substantial tributary, the Afon Gwili, at Abergwili. The estuary meets Carmarthen Bay east of the Pendine Sands along with the River Taf and both branches of the River Gwendraeth. The estuary was guarded by Llansteffan Castle, a 12th-century Norman castle. Damming of the Towy About from its source, the swift flow of the Towy is interrupted by the Llyn Brianne reservoir, created in 1972 by damming a section of the river to store winter rain for release into the river during dry periods. The reservoir supports the new abstraction at Nantgaredig which supplies a large swathe of south-east Wales with drinking water. The flow in the River Towy would have been unable to sustain such an abstraction were it not for the release of water from the upland reservoir. Ecology Fish The Towy is a national draw for big sea trout (local name sewin), the seagoing form of the brown trout, Salmo trutta. These fish enter the river each spring and early summer to breed in the tributaries. The river is thought to produce more double-figure (10 lbs plus, or about 5 kg or more) sea trout than any other in Britain. Anglers and estuary netsmen have taken these fish to over in weight. In summer and autumn there is also a substantial run of Atlantic salmon (Salmo salar). In May the Towy has a run of the rare and protected twaite shad and allis shad. The Towy also contains brown trout, eels, pike, and a variety of small fish species, and is home to brook lampreys, river lampreys and sea lampreys. The Towy has the distinction of having accidentally produced by far the biggest fish ever taken on rod and line in fresh water in Britain. This was a sturgeon (Acipwienser