Cong Burn, a tributary of the River Wear, was once a watercourse polluted through years of neglect from industry.
Parts of it had also been modified by culverts to free up land for roads and other infrastructure, which can also create problems for river wildlife, including fish.
Although there is still work to do to clean up the burn sea trout – an excellent indicator of water quality – are returning there to spawn.
Fisheries officer Paul Frear said: "There are now more than 20 spawning areas in the burn compared to the handful we’ve seen in previous years."
"This is excellent news and shows that the conditions are right for the fish to return.
"The Environment Agency has paid for members of Chester-le-Street Angling Club to remove a weir and make changes to underground culverts to help fish move along the watercourse.
"Improvements to the quality of the water in Cong Burn have been made following water company investment at Hustledown sewage treatment works."