And while the projects are usually sited away from fish-spawning grounds, aquatic life still can get trapped behind the weirs or at the mouth of the penstock.
He said work could only be carried out during the summer because all weirs had to be fully functional during winter to manage river levels during periods of higher rainfall.
Steering between stone weirs and hidden eddies, the boatman points out wildlife along the riverbanks: shelducks in the shallows, water buffalo in the grass, a grey heron hidden among the reeds.
Now that the operation is being paid for by the Weirs, the existing funds will be set aside to cover the cost of the intensive rehabilitation and physiotherapy needed after the surgery.