The charity aims to build a small weir to channel water down an underground pipe to a 650kw turbine, housed in a building on the trust’s Hafod Y Llan farm near Beddgelert.
It is hoped that the scheme could generate enough power to run all of its Welsh properties, including castles, mansions, gardens, and 200 farms.
The National Trust said it had undertaken numerous consultations with the local community, and conducted surveys of the wildlife, archaeology, ecology as well as the river itself to ensure the proposal has minimal or no impact.
Keith Jones, environmental practices advisor, said:
"The proposed site on Afon Cwm Llan is absolutely perfect because the river is sterile due to its industrial past, and there is a history of hydro electricity in the area.”
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