A Wrexham mill owner says he will fight Environment Agency Wales (EAW) plans to develop a weir on the River Alyn.
EAW says the structures, or baffles, will make it easier for salmon and sea trout to migrate up river to spawn.
Mike Kilgannon, who lives in the mill at Rossett and uses its water wheel to grind corn for his own use, fears the river level and the strength of the flow will be affected.
The agency said his concerns were unfounded and levels will not be hit.
The work on the weir is part of a five-year plan to open up 25 miles (40km) of river for spawning fish on the Alyn.
Salmon and sea trout
Work on the Pont y Capel Weir has already been completed, and is due to start in Rossett and in Caergwrle over the next two years.
In response to Mr Kilgannon's concerns, an EAW spokesperson said: "These concerns are unfounded.
"This work has been designed so it will not affect the level of the water in any way and the mill can continue to operate.
"It will also allow all species of fish, rather than the odd salmon and sea trout, to reach different parts of the Alyn."
The EAW said the current 10ft (3m) weir could "not be considered nature's way", and the new system would allow fish into streams that have been inaccessible since the weir was built in the 1800s.
Mr Kilgannon said he would employ a barrister to fight the plans if necessary.