Current pinned teen underwater

The gateway against which a teenager drowned while swimming in an irrigation waterway in Canterbury's Waimakariri River. Photo / supplied

A young woman who drowned at a popular Canterbury swimming spot yesterday is thought to have been pinned against a gateway by fast-flowing water.

The 18-year-old was swimming with friends in an irrigation waterway in the Waimakariri River at about 1.30pm when she became trapped against the gateway, which feeds a water storage pond.

Selwyn District Council chief executive Paul Davey said her friends tried desperately to save her but the current was too strong.

A council contractor arrived in about 15 minutes and opened the gate to release her body, which was recovered further down stream.

Mr Davey said the waterway, which is near Kirwee in the Selwyn district, was generally considered safe and had been used by swimmers for many years.

"However, like any river there are significant flow variations and at times of higher flow things can get a lot different and that was the case yesterday."

The council was considering what actions to take to prevent further tragedies.

"We've got a problem with warning signs because when we put them up they get vandalised because it's a highly-trafficked area and a very popular recreational site, so we're thinking about whether we need to put up more robust signage that will withstand the rigours of vandalism.

"We can't stop people from swimming in rivers. What we need to do is just to show them the risk of rivers in higher flow because they're a much more dangerous beast than when they're flowing normally."

There was already a warning sign about 100 metres from where the young woman drowned, he said.

Council engineers were looking at whether there needed to be structural alterations to the waterway to make it safer.

"We've also got to remember that this is an important economic waterway for wider Canterbury and we've got to make sure that we don't limit our ability to service the farmers who rely heavily on this water source, but obviously we don't want to contemplate ever having another tragedy of a similar nature.

"We are just so gutted for the loss of a young life and we extend our condolences to the friends and family of this young person, it's just devastating."

Police could not be reached for comment.