Naturalist Iolo Williams and over 1,000 others want to protect iconic Llan Ffestiniog waterfall, Rhaeadr y Cwm.

Campaigners say it is threatened by a hydro-electric scheme.

The formal consultation period closed in September. Over 1,000 individual objections were submitted, many through a web page set up by the Snowdonia Society, Save our Rivers, the North Wales Wildlife Trust and Buglife.

Campaigners say the application concerns a designated SSSI where rare damp-loving plants grow. Removing significant amounts of water risks changing the conditions that make it special, and the amount of renewable electricity generated would be quite small, just 600kW.

Iolo said: “It is unbelievable that Snowdonia National Park have agreed, in principle, with this plan. Rhaeadr y Cwm has been designated as an SSSI due to the association of under-plants that grow in such a wet environment. A draw of nearly 70 per cent of the water would be extremely harmful to the rare ferns and mosses. I'm very supportive of social green energy but not when it means destroying the natural beauty of Wales.”

Cymdeithas Eryri Snowdonia Society director Rory Francis said: “Cwm Cynfal has inspired storytellers, artists and poets over a millennium. This is a landscape associated with the tales of the Mabinogion and also famously depicted by David Cox in 1836 in his iconic painting Rhaiadr Cwm. It is one of Eryri’s most majestic waterfalls, but is being threatened by a hydro-electric scheme that would see the river dammed and at times, just short of 70 per cent of the water diverted out of the waterfall.”

Conservations say they support decarbonisation but there is a need to weigh the damage against the benefits.

A Snowdonia National Park Authority spokesperson said: “A planning application for the development of a hydro-electric scheme in the Afon Cynfal near Llan Ffestiniog has been registered. Planning Officers are currently assessing the application which will be presented to the Planning and Access Committee in due course.

“While the Eryri Local Development Plan does allow, in principle, for the development of small-scale hydro-electric schemes, each application must be assessed on its own merits with safeguards to protect the landscape and visual amenity of the area, the ecological value of the particular watercourse and surrounding habitats, trees and archaeology.

“No decisions are made on any individual applications until planning officers are satisfied that all assessments and consultations have been completed – and this thorough assessment is currently ongoing.”