Boat owners making their way south this summer have been urged to keep an eye out for hazards when on the rivers.
Environment Southland has discovered railway irons used in erosion-control methods several decades ago have found their way into some southern waterways.
Council staff have found the irons in the Makarewa, Ōreti, Mataura and Waikaia Rivers.
Environment Southland said it had worked hard to remove the irons, which so far had been discovered both protruding from river waters and submerged under the water surface.
The catchment operations team removed the irons in several locations.
However, some irons remained embedded in rivers and posed a safety risk.
The maritime team had been out marking sites where irons were discovered with buoys and flags.
Warning signs had also been erected at the sites, Environment Southland said.
Deputy harbourmaster Zak Smith said the council was urging river users to keep an eye out for hazards.
"Anyone boating near the sites where railway irons have been discovered is advised to travel with extreme caution," he said.
"The changing nature of our rivers, in the decades since these railway irons were originally placed there for erosion control, has resulted in the railway irons now being in the main river channel making them a major safety hazard," Mr Smith said.
Environment Southland staff were looking at other sites of interest, he said.
He encouraged anyone who saw irons in rivers to contact them so that they can be marked and removed as quickly as possible.
"It is possible that there are more that are yet to be discovered on other parts of Southland’s rivers, particularly because some have been found submerged under each river’s waterline."
