The Selwyn River is polluted and running dry. (Video first published December 2016)

The signs say there's a river here, but no one's seen it in months.

A long stretch of the Selwyn River near Christchurch is barren. Its dry river-bed is snaked by tyre tracks, faint clues of its past as a river disappearing as it becomes a vehicle track.

A beloved swimming spot downstream is stagnant. Fish and eels die in their dozens, trapped in pools evaporating around them.

IAIN MCGREGOR/FAIRFAX NZ People camp next to Coes Ford, a once popular swimming hole.

It's not new for some parts of the Selwyn to dry up, but the scale of this year's disappearance is unprecedented.

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IAIN MCGREGOR/FAIRFAX NZ Distinguishing where the Selwyn River used to flow is difficult now the water is gone.

The spring-fed river begins in the Southern Alps and seeps underground when it reaches the plains, re-emerging near Lake Ellesmere in a stretch now too polluted for swimming.

Each year the dry expanse in the middle gets longer and stays through the winter. The water that is there is restricted to small, feeble puddles.

There are concerns that this represents a new norm in a changing climate, where waterways are dry, fish need to be rescued in their thousands, and roads and rivers are indistinguishable.

IAIN MCGREGOR/FAIRFAX NZ Dried up silt and flattened plants show where the river flows when it floods.

"Even in the middle of winter you can drive your car across it," said Rod Cullinane, North Canterbury Fish & Game general manager.

"We're going to see the position get worse. There's been a certain amount of recharge from the aquifers over the winter, but that would have all been used up by now."

Last week around 70 eels were found dead on the rocky riverbed, with about as many trout.

DOC/SUPPLIED A longfin eel rescued from the Selwyn River by a DOC ranger last week and released to safety. It was one of about 70 eels, almost all of which died.

Among the dead were endangered longfin eels, massively long and likely upwards of 70 years old, destined for the sea where they breed at the end of their long lives.

Eels still try to migrate up the Selwyn when it rains, using the brief window of water as an escape tunnel, only to become trapped when the river vanishes.

The Department of Conservation (DOC) managed to save one longfin last week. A few more were rescued by locals, after a Facebook post to a community page requested help, but the vast majority died.

IAIN MCGREGOR/FAIRFAX NZ Coes Ford, a popular camping spot, is almost stagnant.

Where the eels were found, the old edges of the riverbank can be seen above a path of dried silt and flattened weeds, like the river's shadow.

The low flow has had a disastrous impact downstream, at the once popular swimming spot Coes Ford.

On Wednesday, the last day of Spring, Coes Ford was a trickle of ankle-deep water buried in weeds protected by a wall of mosquitos.

ECAN/SUPPLIED Flow data for the river at Coes Ford. It has reached record lows.

It was a good swimming spot as recently as two years ago.

Data shows the flow at Coes Ford fell to a record low in September. The flow has halved since then and is now a sixth of its usual flow.

A long time ago parts of the river were so deep you could dive from the willows lining its banks, but now those same stretches have no water at all.

IAIN MCGREGOR/FAIRFAX NZ Algae in the river

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"Dry conditions, particularly the lack of rainfall in the past three winters, is the main reason," said Environment Canterbury (ECan) scientist Adrian Meredith.

"Extraction for irrigation also contributes."

IAIN MCGREGOR/FAIRFAX NZ Wild flowers grow where the river once ran.

The region has a deficit of water due to prolonged drought conditions.

One part of the Selwyn district had 42 per cent of its usual rainfall between June and September. Last year brought record low rainfalls.

"Although there has been rain in the last few months, it has not been enough to fully recharge aquifers.

IAIN MCGREGOR/FAIRFAX NZ Dried silt on river stones.

"As a result, when flows from the headwaters increase after rainfall, the water sinks into the gravels rather than flowing at the surface."

Some are not convinced.

Fish & Game said there was a clear connection between extraction for irrigation and dry rivers than authorities would care to admit.

The amount of groundwater allocated for irrigation in Selwyn is 134 per cent of the limit.

Some irrigators have been on restrictions, particularly around Coes Ford, but many irrigators around the river were in action this week.

Fish & Game was now using its resources to rescue fish in their thousands throughout Canterbury.

It did not agree that weather was the main culprit.

"Fish & Game doesn't hold that view nor do an awful lot of other people out there," Cullinane said.

"We salvage literally thousands of fish from the Ashley River [in North Canterbury] each year. It sucks up a lot of our time and energy doing that."

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