Work is planned to repair a weir and build a new fish pass on the River Tees, the Environment Agency has said.
Erosion by the river has caused the concrete structure, built in 1963 near Barnard Castle, to break up in places.
It is used by the Environment Agency to measure the depth and flow speed of the river which is used to provide flood warnings in Yarm and Neasham.
The fish pass will help salmon, sea trout and other smaller species move upstream.
The work is expected to start in the next two weeks and will be carried out over the next two years.
Jenny Evans, from the Environment Agency, said: "We are doing this work to ensure that we can continue to provide flood warnings for the people living and working in Yarm and Neasham."