Canvas Print of The River Tywi in the Tywi Valley in rural Carmarthenshire in January. It is about 75 miles long running from the Cambrian Mountains to the sea. The river is popular with fishermen and bypasses Llandovery, Llandeilo and Carmarthen on the flat valley. Some information about the River Tywi. The Tywo Valley has so much of interest, including castles, a folly tower and Gelli Aur or Golden Grove, an old mansion house.