Time is running out to protect Franz Josef from its rising river. Stopbanks won’t work forever

Each year, more than a million visitors pass through the small South Island township of Franz Josef/Waiau on their way to see the West Coast’s glaciers and world heritage sites.

But the town faces a rising flood risk because it sits beside one of the country’s most dynamic rivers.

The Waiho River carries enormous quantities of gravel from the Southern Alps. As it leaves the mountains, it would naturally deposit gravel across a wide area, changing course over time.

But since the 1980s, stopbanks (artificial levees) have confined the river to about a third of its natural bed, causing gravel to build up instead of spreading. As a result, the riverbed is rising by about 20 centimetres a year, three times faster than it would naturally.

In 2023, as members of the independent Waiho Technical Advisory Group, we warned that the rapidly rising riverbed posed an increasing threat to the town.

We recommended a ten-year plan centred on giving the river more room to move and deposit its gravel, rather than relying on ever-higher stopbanks.

The physics is simple: feeding the same amount of gravel into a smaller area makes the riverbed rise faster. As the riverbed rises, the safety margin provided by the stopbanks shrinks.

When the group reconvened in August this year to review progress, we found the river was changing faster than the plan was being implemented.

Time is running out fast. Keeping the town safe will require an approach to river management that offers lessons for other communities in New Zealand and around the world facing similar challenges.

A river finding a new path

Since 2010, the Waiho has increasingly spilled north into the neighbouring Tatare Stream, which is about ten metres lower. From January 2023, this spill has carved out a new channel capturing ever more of the Waiho flow.

Between March 2025 and March 2026, the spillover point moved a kilometre upstream and the new channel cut down by two metres, exceeding modelled predictions.

Erosion along the new channel has already undermined the base of Havill’s Wall, which protects the town’s wastewater treatment ponds. But the Tatare channel is also filling with sediment. Once it is full, possibly within a decade, the Waiho is expected to reoccupy its southern bed.

Large quantities of gravel also remain upstream, ready to move during future storms.

An earthquake along the Alpine fault will compound the problem, damaging stopbanks and triggering landslides that deliver additional gravel into the river, accelerating the riverbed rise.

The uncomfortable solution

The obvious response to a rising riverbed has been to build higher stopbanks. But that treats the symptom, not the cause. While the river remains confined, its bed will continue to rise rapidly and the banks will need to keep pace.

Recent stopbank raising has temporarily reduced the likelihood of flooding, but not its potential consequences. With the river sitting higher above surrounding land, any breach will release floodwater with catastrophic results.

Our central conclusion in 2023 was counterintuitive but logical: protecting Franz Josef/Waiau requires giving the Waiho more room.

Releasing the river across its western floodplain would allow gravel to spread over a much larger area, slowing the rate of increase of pressure on the stopbanks protecting Franz Josef/Waiau.

Nobody owns the problem

Making room for the Waiho will not be simple.

It would mean relocating part of State Highway 6, the critical lifeline and main tourist road connecting communities along the West Coast, and removing landfills, farms and other infrastructure on the western flats.

These actions formed part of the ten-year programme we recommended in 2023. Three years on, land purchases and relocation have not started, while highway planning remains at an early stage.

Continuing delays mean stopbanks have to be raised further, with associated costs and increased risks, while the consequences of stopbank failure worsen incrementally.

Part of the issue is governance. Flood protection, land-use planning, managed retreat, emergency management and local infrastructure all sit with different organisations. No single body is responsible for delivering the whole plan.

We propose a new group led by Westland District Council and including the West Coast Regional Council and central government agencies, with a mandate to accelerate the work and pursue government funding.

Franz Josef/Waiau is an early test of a question communities in New Zealand and elsewhere already face, and will face repeatedly in the future. Who leads, and who pays, when relocating property and infrastructure is the safest response to a growing natural hazard risk?

Independent experts have warned since 2002 that the traditional approach of raising stopbanks cannot continue. What is missing is not more information, but ownership and funding. Without both, the river will make the decision for us.

The authors acknowledge the contribution by Matthew Gardner, Director at Land River Sea Consulting and Mark Healey, principal engineer (rivers) at WSP.