Natural Resources Wales says work being carried out is not only protecting people in the Upper Swansea Valley from flooding.
But it will also help to provide the conditions for salmon to breed.
About 500 tons of stones and gravel – called shoal – are being removed from the river bed where the Tawe meets the Twrch.
It'll let the rivers to flow more freely and reduce the risk of flooding to 268 homes and 3 businesses in nearby Ystalyfera.
Up to 35 lorry loads of the shoal will be washed and graded to determine their suitability as an environment for salmon to spawn.
Up to 40 per cent of the stones taken from the Tawe and the Twrch will be “recycled” in other locations as spawning gravel.