Afonydd, cronfeydd dŵr a chorsydd blanced


The Cambrian Mountains form the main watershed of Wales. The rivers Severn, Wye, Elan, Irfon, Tywi, Cothi, Teifi, Ystwyth, Rheidol, and Twymyn all have their sources here.  The river valleys, such as those of the Afon Gwesyn and Rhiwnant, include numerous striking waterfalls, the equals of many “world famous” waterfalls in this country.

The spectacular gorge and waterfall at Dylife was formed by the deep-cutting headward erosion of the Twymyn-Dyfi to capture the uppermost Clywedog-Severn valley. Similarly, the Teifi was first diverted by rapid headward erosion of the river Ystwyth along the line of the Ystwyth fault, and then the Ystwyth headwaters in turn were captured by the Rheidol. The shortened route to the sea from Devils Bridge (10 miles compared with the previous 50 miles to reach the coast at Cardigan) caused the Rheidol to deepen its bed very rapidly, creating the famous waterfall and gorge at Devil’s Bridge.

The water resources of the Cambrian Mountains are abundant and valuable, and their exploitation by humans – to assist in lead-mining, and more recently to provide water and energy to remote towns and cities – has had a major impact on the Cambrian Mountains landscape.

Nature’s scattering of small lakes was augmented in the 18th and 19th centuries by ponds built to store water used by miners in hushing and ore-dressing, and later to power machinery via a system of dams, leats and waterwheels.

cambrian mountains rivers

Elan Valley and its reservoirs

The coincidence in the Elan valley of dramatically glaciated valleys, high rainfall, and low population resulted in Birmingham’s remarkable Elan Valley reservoir scheme, started in the 1890s. Construction of the dams and waterworks had a huge local impact, with the building of a village for workers, railways, and new roads, as well as the flooding of older settlements. Further changes followed during the construction of the Claerwen dam in the 1950s. The presence of the reservoirs has greatly influenced land use within the Elan catchment.

More recently, six hydro-electric generators were installed in the Elan Valley dams, with a total capacity of 4.8MW, yet minimal landscape impact, since the turbines are hidden in the dams, and the cables laid underground.

Link: Elan Valley | Yours to explore.

Llyn Brianne

Llyn Brianne is situated in the mountainous Powys countryside west of Llanwrtyd Wells, some 19km north of Llandovery.

It was constructed during a period of increasing water shortages in west Wales. Despite the need for it, the scheme attracted many objections before a public enquiry sanctioned its construction under the West Glamorgan Water Board (Llyn Brianne) Order 1968. The dam was completed on the headwaters of the River Tywi in 1972.

Described as a regulatory reservoir, its purpose is to supplement flows in the Tywi during dry periods, as compensation for water supplies abstracted some 40 miles downstream at Nantgaredig, near Carmarthen, and destined for West Glamorgan.

Although an attractive large, open body of water, Llyn Brianne is not open for recreational water sports and has no visitor centre, although there is parking at the dam and public conveniences. The track from the dam is a good place to start to explore the surrounding hills; and the grassy slopes provide excellent picnicking sites.

Visitors can also drive to the bottom of the spillway to view the water jet at the base of the dam. It has become significant visitor attraction.

A hydroelectric power station was added 25 years after the dam was completed capable of generating 4.3MW. As part of this project, the dam and spillway height was raised by a metre, making the dam the tallest in Britain at 91m.

A further 1.8MW hydroelectric installation was added a few kilometres downstream of the dam, on the Afon Towy, in 2021.


Llyn Brianne and Rhandirmwyn village

Llyn Brianne and Cilycwm village

Llyn Brianne Dam and Reservoir

cambrian mountains llyn brianne

Nant-y-Moch and Dinas

Nant-y-moch and Dinas reservoirs were built in the late 1950s as part of the Rheidol hydro-electric scheme, the largest of its kind in England and Wales, which can generate 55MW from its four generators.

The 32 acre Dinas Reservoir, which is the smaller of the two reservoirs, is regularly stocked with rainbow and brown trout. The quality of its fish is highly rated.Nantymoch is the much larger and wilder lake (680acres), provides a huge expanse of wild brown trout. For the angler who wants to get away from it all Nantymoch is the place to visit. Both lakes are bank fishing only. Nantymoch is fly only but on Dinas spinning and worming are also allowed.

Link: Rheidol Visitor Centre, Cafe and Power Station

Blanket bogs and rivers

flood management and carbon sequestration


The Cambrian Mountains holds a sizeable proportion of Wales’ deep peats, underlying the blanket bog. These are perhaps the key characteristic of the rolling landscape of the Cambrian Mountains, formed during the Pleistocene ice ages, and now largely clothed in a combination of blanket bog, heath and acidic grassland. Under all of these habitats, but notably beneath the blanket bog, are soils with organic rich upper horizons: essentially, natural carbon ‘sinks’.

Much of the blanket bog coating the Cambrians is part protected under scientific designation (i.e. SAC, SPA and SSSI) and some areas are also under conservation style management, such as that managed by The National Trust on its Abergwesyn Common. On the other hand, the 2020 National Peatland Action Programme 2020 – 2025 recognises the damage to deep peat caused by conifer plantations – which, as part of the Welsh Government’s Forest Estate, cover significant parts of these hills. The Programme includes guidance to remedy the damage to the deep peat underlying such conifer plantations, as well as in considering the (doubtful) need to restock Sitka on areas of deep peat. Nevertheless, the need for such guidance illustrates the inadequacy of the current piecemeal approach for effective long term management of such a large scale but ecologically fragile resource.

Rivers and flood management

Pumlumon alone, is a considerable catchment, covered in large areas of blanket bog and forest, together acting as a sponge for the high rainfall across the area. No wonder that for many years the UK Centre for Ecology and Hydrology has had a major research project based on this important catchment.

The Elenydd, a huge upland block, is sometimes referred to as ‘the green desert.’ Again, it comprises a combination of expansive blanket bog, crossed by mountains streams which merge to form the rivers flowing out of the block, such as the Tywi and Irfon, and bordered by coniferous commercial forestry as well as native deciduous woodland. This block also includes Dwr Cymru’s reservoirs such as the Elan Valley system and Llyn Brianne.


Mynydd Mallaen is a large upland dome clothed in a tapestry of water-retaining heath, unimproved grassland and blanket bog.

With further investment from Welsh Government, via Natural Resources Wales, these hills have the potential to make an ever larger contribution in reducing flooding downstream both in Wales and in England (around the lower reaches of the Severn and Wye).