A stunning film documentary follows the journey of the River Tywi from the estuary in Carmarthen Bay to its source in the heart of the wild, isolated Elenydd mountain range.

The 50-minute film Yr Afon Dywyll – Taith yr Afon Tywi will be premiered on television on S4C on Sunday, 4 November and viewers are in for an unforgettable nature trek following the longest river that runs entirely within the borders of Wales.

The programme follows the 80-mile long journey of the river from the estuary villages of Ferryside and Llansteffan, famous for its Norman castle, past the towns of Llandeilo, Carmarthen and Llandovery, and historically significant castles like Dinefwr and Dryslwyn, through Lake Brianne reservoir to its source in the mountains.

The documentary’s title translated into English is ‘The Dark River – The Journey of the River Tywi’. This refers to one possible meaning of the name ‘Tywi’; one traditional belief is that it originates from the Welsh word for ‘dark’, ‘tywyll’. Some parts of the river meanders through dark, narrow valleys, but the terrain varies hugely, and the landscape also changes greatly over the four seasons.

Producer and director Richard Rees and his team at Telesgop production company captured the fauna and flora of the river and estuary over a period of filming throughout a number of years. The jaw-dropping array of wildlife captured on film include grey seals, camera-shy badgers, red deer and foxes, birds such as wild and mute swans, migratory birds such as dippers, swallows, wagtails and various breeds of geese, and fish such as sea lampreys, sewin and salmon.

“One animal which can attest to the ecological health of the river is the otter, which was a rare sight in the river 50 years ago. Thanks to carefully targeted conservation schemes, this most engaging of river creatures is flourishing on the river,” said the award-winning wildlife film producer, Richard Rees.

People featured in the programme include sea angler Enoch Rees, river fisherman Aled Davies, cattle farmer Tony Jones and artist David Cowdrey, all of whom live and work on the river’s banks. However, some of them express concerns about the ecological future of the Tywi, particularly the reduction in some species of fish, especially salmon, and some experts suggest that this has been caused partly by agricultural pollution which creates algae blooms that kill river life.

“In the 10 years between 2006 and 2016, a total of 490 cases of pollution were recorded on the River Tywi, silage effluent and sheep dip have been the main culprits. With two of every three UK rivers failing to reach the ecological standards and river life continually deteriorating, now is the time to safeguard our rivers before it’s too late so that wildlife and future generations can enjoy them,” said Richard Rees.

Yr Afon Dywyll - Taith yr Afon Tywi will be shown on S4C on Sunday 4 November at 8pm. S4C

English subtitles and on demand service available.