The Selwyn River near the Selwyn Huts used to be a popular swimming hole, but is now polluted and unsafe.

A tiny, century-old community built around a river can no longer swim in it because it has become too polluted.

A rope swing dangling uselessly by Canterbury's Selwyn River is a reminder of a better time in the tiny community of Selwyn Huts – when families spent long summers in the river, before it became too polluted for swimming.

The lower stretches of the river have become toxic and shallow. The water is so green it glows in the sun.

STEVE CURTIS/SUPPLIED The water is permanently discoloured.

For about a century, people swam and boated in the river, southwest of Christchurch, but doing so now could make you sick.

"It's disgusting. It really is," said Steve Curtis, who lives opposite the river.

"We used to have boat races every year. Now that's out of the question."

SUPPLIED A photo taken in 1985 shows children playing in clear water.

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It only took a couple of decades for the popular swimming spot at Selwyn Huts to collapse.

Its impact on the tiny community – comprising about 100 ramshackle huts near the riverbank, some nearly a century old – has been significant.

SUPPLIED Another photo from 1985 shows clear water.

Health warning signs are older than some of the local children. Traditions going back decades have been abandoned.

Their once swimmable river is now only wadeable; in line with Government standards, but certainly not their own.

Last Christmas, for the first time, the river was empty. Families stayed out of the river and played games on a nearby tennis court instead.

CHARLIE MITCHELL/FAIRFAX NZ The river is green and still.

"Over Christmas time it was just still and stagnant," Curtis said.

"My little kids were out swimming and boating and everything in the river. You just wouldn't do it now."

Andre Brocherie remembers jumping into a cold, clear river. He has lived at Selwyn Huts since he was aged 2.

JOSEPH JOHNSON/STUFF An unused rope swing at the Selwyn River. The water is too polluted for swimming.

Now 32, he is disgusted by what the river has become.

"As you can see, no one uses it," he said.

"It's not enjoyable. You don't even want to take a photo of it anymore."

SUPPLIED A solitary trout in the river once famous for its teeming trout population.

A few weeks ago, he took photos of a brown, effluent-like sludge creeping up the river like an oil spill.

"There's a good chance of getting sick. You always have to have a shower straight after.

"It's not like the old rivers, which you could have a bath in."

Late last year, signs protesting the poor health of the river appeared.

They were put up by local artist Mike Glover, who had become increasingly upset with the deteriorating river.

"I thought of my daughter, who had just turned 10, and thinking that this is just not acceptable," he said.

The river was once the "absolute hub" of the community before it withered. His signs were just one attempt to make his frustration clear.

"I kept getting madder and madder, basically."

He painted the word "permanent" over a temporary health warning sign. If nothing else, at least the signage was now accurate, he said.

The Selwyn River was once one of the world's greatest trout fisheries.

In the 1960s, the trout population reached 65,000, enough to stock every river in the South Island.

There are now just several hundred trout in the river. Those remaining are thin and inedible due to the river's health.

The river's problems were the result of "poor incremental decisions" over several decades, North Canterbury Fish & Game environmental adviser Scott Pearson said.

Intensive farming had become too widespread in the area, and too much water was being extracted for irrigation.

"These decisions can appear small on their own, but when you add them all together and do that over several decades, you suddenly have a real mess on your hands," Pearson said.

While groups including Environment Canterbury (ECan) were doing good work to improve the river, fixing the problem would be "like turning the Titanic around."

ECan was not available for comment on Sunday, but recently changed its land and water plan to include a raft of measures to improve the river's health.

They included setting new water allocation limits, preventing new water takes, and restricting the transfer of water use permits.

There were also new rules around stock exclusion.

All of those measures would likely improve the river's water quality, ECan surface water science manager Tim Davie said earlier this year.

He said that intensive land use had affected the river's water quality.