The Afon Tywi (also known as the River Towy) is Wales' longest river entirely within the country, running approximately 75 miles (120km) from its source in the Cambrian Mountains above Llyn Brianne reservoir to the sea at Carmarthen Bay. The river passes through the towns of Llandovery and Llandeilo before reaching Carmarthen, one of Wales' oldest settlements, which sits at the point where the river becomes tidal.

The river offers a variety of paddling experiences along its length. The upper sections include gorge paddling with rapids and more technical whitewater features, where rocky cliff sides and steep hillsides characterise the scenery. One notable feature in the gorge section is a significant drop into a gorge, where the river merges perpendicularly below the fall, presenting a challenge for less experienced paddlers. The recommended line at this rapid is to the right, though this route is too narrow for canoes. The lower sections, such as the run from Nantgaredig to Carmarthen, are more gentle in character, featuring oxbow lakes, sandbanks, and a transition into tidal water as the river approaches Carmarthen. Paddlers should be aware that sandbanks in the oxbow areas can be shallower than they appear.

Before paddling, water levels should be checked at the gauging stations on the river, particularly those at Tregaron Castle and Capel Dewi, using the River Levels UK website. On the Nantgaredig to Carmarthen section, a reading of around 1.1 metres at these two stations represents conditions well within the normal (green) range. There is no formal right of public navigation on the non-tidal sections of the Tywi, and paddlers should be aware of this legal position.

Anglers use the river, though the fish stocks have reportedly declined significantly in recent years, meaning encounters with fishing activity are now relatively infrequent. The river corridor supports wildlife including otters, seals, and rare bird species, particularly in the lower and estuarine reaches. The landscape along the river includes several historic landmarks, most notably the ruins of Carreg Cennen Castle near Llandeilo and Carmarthen Castle, which is visible from the water as paddlers approach the town. Carmarthen itself has Roman origins, with a fort established around AD 75 and one of only seven surviving Roman theatres in Britain located there.