Approximately 170 years ago, British settlers initiated the construction of Christchurch city, largely overlooking the adjacent Waimakariri River, which flows from the South Island’s Southern Alps toward the eastern coastline.
However, natural events such as rainfall and glacial movements prompted the braided river, a rare type of watercourse characterized by multiple intertwining channels, to reshape itself, leading to periodic flooding and the deposition of significant amounts of shingle.
By the 1920s, authorities labeled the Waimakariri a “flood menace,” indicating a “deficiency of nature that must be corrected by human intervention.”
This led to the river being contained through the construction of stopbanks, the introduction of non-native tree species, and gravel extraction. As a result, ongoing maintenance is now essential to manage the river and mitigate flood risks to residential areas, infrastructure, and the nearby airport.
Fred Brooks, a river engineer with Environment Canterbury, noted, “People argue against interfering with the river, but the consequences of inaction could be far worse. Given the extent of prior interventions, continuous management is now necessary.”
The Waimakariri is among roughly 150 braided rivers in New Zealand, with 60% located in the Canterbury region of the South Island. These unique river systems are also found in a limited number of locations globally, including Alaska, Canada, and the Himalayas.
These rivers face numerous challenges, often exacerbated by encroachments for agriculture and urban development that disrupt ecosystems, degrade water quality, and increase flood risks for adjacent communities.
Growing concerns about the sustainability and resilience of braided rivers are leading to discussions on how to coexist with these waterways while preventing further ecological decline. Jo Hoyle, a river geomorphologist at Earth Sciences New Zealand, remarked, “Braided rivers are iconic, and we often reference them, but are we truly protecting them?”
In contrast to single-channel rivers, braided rivers are highly dynamic. They originate in alpine regions, rapidly descending to plains where they transport gravel and create channels that branch out into multiple flows. These rivers can shift their paths dramatically, often reverting to previous routes following heavy rainfall.
Over time, the braided rivers of Canterbury have been intentionally narrowed. Gravel extraction for flood management and road construction, as well as water diversion for intensive dairy farming, have significantly altered their natural state.
In the Waimakariri, gravel is regularly removed to prevent the river from breaching its banks and flooding thousands of homes.
As a result of these interventions, the Waimakariri faces challenges in reverting to its original form. The question now arises for other rivers in the region: should they be allowed more space to flow freely?
“It’s a complex question,” Hoyle stated while surveying a narrow channel at the edge of the Rakaia, another major braided river south of Christchurch. “It’s not viable to simply let rivers wander freely. We’re exploring how much space they need to function naturally, support wildlife, and flood without causing excessive damage.”
When a river shifts its course, it often leaves behind fertile land that landowners quickly claim, a phenomenon known as agricultural encroachment. If the river attempts to revert to its previous path, landowners may construct barriers to prevent this.
“This results in a continual narrowing of the river,” Hoyle explained. “Our research indicates that nine of Canterbury’s rivers have narrowed by an average of 50%, with some sections experiencing over a 90% reduction.”
While landowners are permitted to encroach upon braided riverbeds when water recedes, scientists and advocates are calling for changes to this practice.
Deliberate constriction of river channels endangers wildlife and increases flooding risks for communities, according to Hoyle, who advocates for exploring managed retreat options. “The land adjacent to these rivers is valuable, but it is also highly susceptible to severe flooding,” she added.
Emerging issues beneath the surface of braided rivers have also come to light, as communities report declining fish populations and increased pollution in their fishing nets.
The Rakaia River, known for its salmon fishing, has become a focal point for local activity, with a prominent 11-meter-high statue of a salmon serving as a town landmark. This year, however, the annual salmon fishing competition faced an unusual restriction: no fishing was allowed.
“Fish numbers are dwindling,” remarked Chris Agnew, president of the competition, while navigating his boat at the river mouth, where birds flew against the backdrop of a sunset.
According to Fish & Game, only 608 salmon were recorded in the Rakaia during the 2024-25 season, a drastic decline from over 20,000 in 1996.
Scientists continue to investigate the causes of this decline, with potential factors including rising ocean temperatures, sediment accumulation, pollution, and altered water flows affecting breeding patterns and fish behavior.
Frances Schmechel, biodiversity manager at Environment Canterbury, noted a decline in river bird populations, attributed to invasive plant species that provide shelter for predators, and the proliferation of exotic willow trees that inhibit natural river dynamics.
Stokell’s smelt, a small native fish that once thrived, is now classified as nationally critical.
Bruce Kelly, a local angler with four decades of experience fishing the Rakaia, expressed concern: “In the past, even when you didn’t catch any fish, you’d at least see a few. Now, they seem entirely absent.”
Agnew fears for the community’s identity, reflecting on the salmon statue: “It might one day serve as a monument to the loss of what we once had.”
