Some spring evenings, when the tide surges up the mouth of Aotearoa New Zealand’s Waikato River, you’ll spot glistening streams of tiny, transparent ‘glass eels’ making the critical transition from saltwater to sweet.
These juvenile long- and short-finned eels (Anguilla dieffenbachii and Anguilla australis) have traveled a long way to get there.
Both species breed and spawn in tropical waters near Tonga, over 2,000 kilometers north of the temperate rivers where they spend most of their lives. As they journey further upriver, they become elvers: opaque, blue-eyed, mud brown.
Called tuna in Māori, these creatures are keystone species for many local iwi (tribes): an indicator of river health, a prized food source and a character in scores of stories and songs.
Yet many of them won’t make it to the upstream pools they’re seeking to settle in – or back to the tropics to reproduce at the end of their life cycles, which can happen up to 100 years later.
The Waikato is New Zealand’s longest river, and the Waipā its largest tributary. Their waters spill clear and ice-blue from their respective headwaters in Lake Taupō and the Rangitoto Range, yet by the time they reach the ocean, they’re often silty and brown with pollution from farms, towns and factories.
The Waikato’s flow is also disrupted by a sequence of eight hydrodams, with the lowest, the Karāpiro Power Station, being the first to obstruct the elvers’ journey upstream.
Elvers can scale waterfalls and even survive for days out of water. But giant, vertical concrete structures – the Karāpiro is 52 meters high – remain a formidable challenge, and most that try to climb them die in the attempt.
Others stay put just below the dam.
“Often, you’ll find there’s a whole bunch of predators there that will just sit there and eat them while they’re stuck there, because their behavior and motivation is driven by that desire to move upstream,” says Paul Franklin, a freshwater ecologist at the National Institute of Water and Atmospheric Research (NIWA).
“So they’ll keep going until they hit a barrier like that, and they will keep trying for some time.”
Now, a multi-iwi group – the Waikato-Waipā River Iwi Tuna Collective – is intervening to give those elvers a literal boost: specifically, catching them below the dam and translocating them further upstream. They’re also helping adult eels navigate these obstacles on their reverse journeys out to sea.
Whilst such manual trap-and-transfer processes are labor-intensive, “as long as you’ve got people who are committed to doing it, they can often be more reliable than some of the mechanical solutions,” says Franklin.
“And I think that the manual nature of the work is actually building people’s connection with the species, too.”
The work begs a bigger question: how do we restore river ecosystems in a context where we also need things from them as a society?
“It’s not realistic to think that we can turn back the clock and restore all of our waterways to what they used to be, because they sit in this broader socioeconomic context,” says Franklin.
“Figuring out ways that we can sustainably balance those different needs is one of the big challenges: it’s knotty, but important – and it’s very different from having a national park and putting it away. There are lots of different competing demands.”
Dams, however – those ambitious large-scale development projects that were widely seen as emblematic of ‘good green growth’ from the end of World War II to the early 2000s – are falling out of favor as their environmental and social costs become increasingly clear.
A recent global review found that they’re significantly harmful to migratory river species: the fish, eels, crustaceans and snails that need connected waterways to fulfill their life cycles.
The review also found that workarounds like ‘fish passes,’ which help aquatic species to bypass barriers through networks of stepped pools, channels, tubes, ramps or even mechanical elevators, produced consistently poor outcomes.
“Despite their widespread use, fish passes often underperform, particularly when designed without understanding the specific behaviors and traits of local species,” said University of Tasmania freshwater ecologist and review co-author Jia Huan Liew in a press release.
“Dam removal, while costly and sometimes constrained by societal needs, remains the most consistently effective strategy for restoring connectivity.”
That’s happening now in a number of locations, particularly in North America and Europe, where many dams are over 50 years old and their repair costs are becoming prohibitive.
At the same time, solar and wind power are becoming increasingly affordable, alongside better storage technologies and advances in energy efficiency and grid management.
This means there are now alternative options for reliable renewable electricity that cause less environmental damage.
Last year, for instance, saw the world’s largest-ever dam removal along the Klamath River and its tributaries in California and Oregon in the western United States.
The Klamath used to be the third-largest salmon-producing river in the country, and Indigenous and local communities relied on its stocks for their diets and livelihoods.
But after four huge hydrodams were built between 1918 and 1964, salmon stocks plummeted.
A salmon’s life cycle is kind of the reverse of an eel’s: it’s born in freshwater streams, migrates to the ocean to live out most of its adult life, and then returns to those streams to spawn and die.
The dams made that passage impossible. The impacts on the ecosystem – and its human communities – cut deep.
Now, after decades of advocacy led by local Indigenous groups, 676 kilometers of waterways are free, and a vast restoration effort is underway on the 890 hectares of land that had been submerged underneath the dam reservoirs. Chinook salmon have been found in the river’s upper reaches for the first time in decades.
The lengthy, expensive and complex process represents both a hopeful story and a cautionary tale for leaders considering new large-scale dam projects.
“Dams were never meant to be pyramids,” Ann Willis, California director of the NGO American Rivers, told Mongabay.
“They’re just infrastructure, and eventually, infrastructure ages. You can either be proactive about repairing, retrofitting or removing it, or you can deal with the far greater costs of a catastrophic failure after it happens. But there’s no question that one day it will fail.”
Despite new awareness of their costs and challenges, new large dam projects are still being proposed and implemented around the world.
This is particularly the case in Africa, Asia and Latin America, where many countries are in the process of substantially expanding their energy grids.
International development banks, as well as Chinese state-owned enterprises and Brazilian construction firms, often play major roles in financing and constructing these dams.
There’s a long history of opposition to their efforts, particularly by Indigenous and other local communities that rely on free-flowing rivers and want to preserve them.
A particularly poignant example is the case of the proposed Agua Zarca Dam, originally a joint project of Honduran company Desarrollos Energéticos SA (DESA) and Chinese state-owned enterprise Sinohydro – the world’s largest dam developer.
Together, they dreamed up a dam on the Gualcarque River, which begins in the tropical montane forests of Intibucá and flows through the biodiversity-rich territory of the Indigenous Lenca people.
If constructed, it would cut off these people’s supplies of water, food and medicine and violate their customary stewardship rights – as well as impact the water security of those downstream.
Without consulting the Lenca, DESA and Sinohydro began building the dam in 2012 and blocked access to the river in 2013.
Led by Lenca woman and 2015 international environmental Goldman Prize winner Berta Cáceres, local communities led a series of roadblocks and other protests, managing to keep construction equipment out of the dam site and delaying the advancement of the project for years on end.
But Honduras holds the dubious honor of being one of the most dangerous countries for environmental defenders, and the Lenca’s story is a case in point.
One protester, Tomas García, was shot dead by Honduran soldiers during a peaceful protest at the dam office in 2013, and Cáceres was assassinated in her home in 2016. Numerous other protesters have been attacked, detained and tortured.
Their efforts were not in vain, at least: key investors withdrew their funding citing human rights concerns, and the project has since been suspended. In 2022, a former DESA executive was convicted and jailed for orchestrating Cáceres’ murder.
A few months before her death, Cáceres featured in a moving documentary, Mother of All Rivers.
“When we started the fight for the river, I would go into the river and I could feel what the river was telling me,” she recalled, sitting on a rock in the river with her children.
“I knew it was going to be difficult, but I also knew we were going to triumph – because the river told me so.”
From the slick of migrating eels traversing the Waikato to the freshly-freed waters of the Klamath and the contested currents of the Gualcarque, it’s clear that rivers are so much more than service providers.
They are living systems, lifelines, political and spiritual entities – and crucial pathways for countless species on the move.
As we grapple with the complex mathematics of energy needs, development and climate resilience, we also need to consider the cost of blocking life in its flow.
That requires, as Cáceres reminds us, learning to listen: to science, to Indigenous knowledge and to the rivers themselves.
When we do, we may find better ways to meet those needs, without damming the future of the creatures and communities that depend on them.
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