The fence building project will cost about £18,000
Sheep and cattle are to be stopped from grazing a river bank in an attempt to improve and restore the spawning habitats for salmon and brown trout.
The Environment Agency is working with landowners near Tregaron on an £18,000 scheme to build new fences at the top end of the River Berwyn in Ceredigion.
The agency said grazing was eroding the bank and was allowing silt to enter the river.
Officials said improved vegetation would provide better cover for fish.
Steve Carter of Environment Agency Wales said: "The River Berwyn is a very important spawning ground for both salmon and brown trout populations within the River Teifi catchment area.
"The improvement of river habitats for salmon and trout will also provide ecological benefits for many other wildlife species for years to come."