Stephen Midzi, the biodiversity conservation manager of South Africa’s iconic Kruger National Park, wants you to know that he’s not an “anti-dam person.”

“Don’t get me wrong,” he says, adding “but, I also think it is a good thing to allow rivers to be what rivers should be.” Midzi is an advocate for free-flowing streams that offer unfettered connectivity for aquatic and terrestrial biodiversity.

But from as early as 1911, Kruger, one of the largest protected areas in Africa, has not been managed that way. Over the decades, park supervisors tinkered with the preserve’s streams and aquifers, trying to improve on what nature had always done and cater to the park’s animals with a copious, assured water supply.

Over time, 97 concrete dams, weirs and earthen dams were constructed and borehole-fed catchments drilled. But this closely spaced and evenly spread water supply didn’t bring about an African Eden. Instead, it caused severe overgrazing, veld degradation and erosion.

Catchment basins and stagnant waters behind dams silted up and accumulated hippo dung, nurturing cyanobacteria and poisoning the animals that drank the water. Species that prefer habitat close to water, such as zebra and wildebeest, flourished. Those that prefer drier areas, away from water where there are fewer predators, less competition and trampling — animals like sable and roan antelope — floundered.

Over the past two decades, people and nature worked to undo the damming; 42 dams inside Kruger were breached and demolished mechanically or by floods.

Today, “Restoring river connectivity is a critical focus,” says Eddie Riddel, the park’s aquatic biodiversity manager. This turnaround in policy is in line with increasing global criticism of one of humankind’s oldest tools for securing water supplies.

Over the centuries, dams — and massive, well-funded, government-supported dam-building initiatives — have been used to manage floods and provide water for drinking, crop irrigation, industry and power generation. But, just as was learned in Kruger, damming benefits come at a price.

As the number and size of dams being built across the world exploded in the 20^th century, and into the 21^st, the ecological, social and economic costs rippled far beyond local dam sites, having regional and global ramifications.

Humanity’s large-scale effort to dam the world’s rivers has been described as the largest single anthropogenic alteration of the freshwater cycle, with dams now helping edge us closer to transgressing a number of critical planetary boundaries, with adverse impacts on biodiversity, the climate, land use, and freshwater. Dams, combined with all the many other human pressures on nature, are helping upset the balance of Earth’s critical operating systems and could endanger civilization, humanity, and even life on Earth as we know it.

Our obsession with dams

The construction of large dams (defined as those storing more than 100 million cubic meters, or more than 3,500 million cubic feet, of water) peaked in the 1960s, with the cumulative volume of water impounded peaking the decade after. Half the world’s large dams were built for agriculture and offer water for 30-40% of the 2.71 million square kilometers (1.05 million square miles) of irrigated crop and grazing land worldwide.

There are thought to be close to 60,000 large dams, storing about a sixth of the globe’s total annual river flow to the oceans. Over and above that, there are at least 16 million small dams and impoundments with reservoir surface areas larger than 100 square meters (1,076 square feet), totaling around 306,000 km2 (nearly 120,000 mi2), which increases Earth’s terrestrial freshwater surface by more than 7%.

South Africa is one country that exemplifies the benefits of dam construction. With low annual rainfall (just 55% of the global average) and high temperatures and evaporation rates, a mere 9% of precipitation that falls here ever reaches rivers — the country’s only large-scale freshwater source. Not surprisingly, the nation decided to rely on dams to store water and unlock South Africa’s development potential.

Today, an estimated 500 large dams store millions of liters of water and allow for activities otherwise near-impossible in the semi-arid climate. The major dams are able to store about 50% of the mean annual runoff, ensuring supply throughout the year. Major cities like Durban, Johannesburg and Cape Town all rely on dams for their water. Climate change recently sounded a warning, however, when drought took Cape Town to the brink of Day Zero in 2021, forcing a much-needed reevaluation of the region’s water system toward becoming a water conservation role model for the world.

Our disappearing aquatic biodiversity

The construction of large dams and reservoirs has slowed globally since the 1960s and ’70s, but much larger rivers are now being dammed. Only 37% of rivers longer than 1,000 km (620 miles) remain free-flowing over their entire length today, and a mere 23% flow uninterrupted to the sea. Counting all hydropower dams planned or being built, natural hydrological flows will be altered for 93% of river volume worldwide by 2030.

And with those dams will come massive aquatic biodiversity loss. “A major impact of the fragmentation of rivers [by dams] is the decline of freshwater species,” says Michele Thieme, WWF’s lead freshwater conservation scientist.

Freshwater species are in rapid decline planetwide. In its “Living Planet Report 2020,” WWF monitored 3,741 freshwater populations worldwide (representing 944 species of aquatic mammals, birds, amphibians, reptiles and fish), and found an 84% average decline in freshwater population size since 1970. Freshwater amphibians, reptiles and fish are the worst impacted, with about a third of all freshwater fish species threatened with extinction, while 80 species have already vanished.

There’s plenty of blame to spread around: Freshwater river systems are where we base our civilizations, where we build cities, roads, industry, and grow our food. “There are multiple levels of interactive impacts on freshwater systems that make it difficult to point to just one [cause of harm] alone,” Thieme says. Habitat modification, invasive species, overfishing, pollution, poor forestry practices, disease and climate change all play a part. However, she adds, the impact of dams on rivers, and the loss of connectivity, are a huge known contributor.

Dams destroy aquatic connectivity

Most directly, dams block the migration of fish and other aquatic species, separating them from breeding grounds and reducing population sizes. Migratory fish populations — including sturgeon, salmon, hilsa and gilded catfish — have fallen by 76% since 1970. In Brazil, dams and other causes are endangering the Amazon’s giant catfish; on Asia’s Mekong River, fewer than 100 Irrawaddy dolphins may remain as proposed dams loom.

Iconic fish, like the beluga sturgeon and the Mekong giant catfish, are also in danger. The world’s largest freshwater predatory fish, the Chinese paddlefish, is already gone. Officially declared extinct in 2020, a paper by Chinese scientists concluded that the “Panda of the Yangtze” disappeared after 200 million years due to the combined effects of overfishing and the disruption of migration routes by both small and large dams.

People, too, are paying a very high price due to lost connectivity. The hilsa fishery once made up the majority of catches in India’s Lower Ganges, where many people rely on freshwater fish as their primary source of protein. Since the 1970s, following the construction of the Farakka Barrage that likely prevented fish from reaching their spawning grounds, catches declined by 94%. Upstream of the barrage, the annual catch dropped from 19 metric tons to 1 metric ton after construction.

Beyond the physical barrier, dams result in vast changes to ecosystems and the life-forms that depend on them. Downstream water temperatures change as water is released, and the natural ebb and flow of the hydrological cycle is altered.

Downstream sites, including lakes and estuaries, are also impacted by the reduced flow of phosphorous, nitrogen and silicon trapped behind dams. Nitrogen and phosphorous nutrients trapped in stagnant reservoirs can trigger algal blooms, eutrophication and massive fish kills.

Dams and the cessation of sediment flow

Another important harm that needs the world’s attention is the disruption by dams of sediment flow, says Thieme. “The cascading effects of this are not always considered, but it has real, global implications.”

According to some estimates, 25-30% of Earth’s land-to-ocean sediment flux is trapped behind dams. Though the science behind the numbers is complex, it’s easy to see the impact of reduced sediment flow on the livelihoods of people living in Earth’s deltas today. Deltas are landforms created by the deposition of sediment carried downstream by rivers as they enter an estuary or ocean.

While ancient civilizations like those of Egypt and Sumeria flourished in these resource-rich environments for thousands of years, a recent study found that, today, at least 25 million people live in sediment-starved deltas. Dams upstream prevent nutrient-rich sediment from ever getting to the deltas, resulting in the loss of large tracts of fertile land to subsidence, erosion, flooding and sea-level rise.

An example is the biodiversity-rich Mekong Delta in Vietnam, the world’s third-largest delta, home to nearly 20 million people and key to Southeast Asia’s food security. Large dams have already been constructed on the Mekong, and more are in the pipeline.

Sediment flow to the Mekong Delta will be reduced by an estimated 97% by 2040, with expected major damage to river’s productivity, geomorphology and persistence of the delta landform itself. Though sand mining contributes, the bulk of Mekong sediment loss is attributed to dams.

Free-flowing rivers transport carbon in the form of organic matter and sediment from upland headwaters, through watersheds to the sea, moving as much as 200 million metric tons every year. But dam disruptions could decrease the export of organic carbon to the oceans by an estimated 19% by 2030, with major potential repercussions for freshwater and marine ecosystems.

Dams, deforestation and climate change

The construction of new hydroelectric dams, access roads, and transmission networks in remote areas can initially cause significant deforestation. But that’s just the beginning: Cheap, government-subsidized hydropower attracts energy-intensive, ecologically destructive industries, such as bauxite mining and aluminum smelting and industrial gold mining. This widespread, often dam-triggered, industrial infrastructure expansion has severe impacts on ecosystems and species — exacerbated in developing nations where environmental regulations are weak.

In the Brazilian Amazon, every kilometer of legal road built through wild areas is typically accompanied by 3 kilometers of illegal roads, resulting in significant forest fragmentation, giving access to wildlife traffickers and illegal loggers, causing roadkill, and allowing more traffic into sensitive areas and attracting settlers. Dams, and the roads that accompany them, significantly diminish diversity.

Though commonly promoted by governments, construction companies, big banks and international investment firms as a clean source of green energy, hydroelectric dams located in tropical regions can accentuate climate change significantly.

Tropical hydroelectric plants and their reservoirs can emit two to three times more greenhouse gases than natural gas, oil, or coal plants, due to deforestation and potent methane emissions.

Rapid, ongoing rot of submerged vegetation in equatorial heat turns reservoirs into major emitters of methane — a greenhouse gas many times more powerful than CO₂. Despite that scientific fact, the U.N. still considers dams a clean source of energy, fails to count reservoir-caused emissions or deforestation in national greenhouse gas totals, and offers carbon credits for new dam construction.

Dams, the carbon cycle and climate change — much to learn

In recent years, researchers have begun overthrowing past assumptions about dams, concluding that they impact Earth’s climate in complex ways through the modification of the carbon cycle and accompanying greenhouse gas exchanges.

“The general opinion was that [dams] store more carbon than they emit,” explains Matthias Koschorreck, a biologist in the Department of Lake Research at the Helmholtz Centre for Environmental Research, Germany. Koschorreck was part of a research team that recently published a paper turning the green status of dams on its head. Their work analyzed the influence of drawdown areas — the edges of reservoirs exposed to air when water levels drop and what looks like a bathtub ring appears, extending around the entire body of water.

“Our study shows that [dam carbon] emissions are much higher [than previously thought],” Koschorreck says. “On a global scale, reservoirs emit more carbon to the atmosphere than they bury in the sediments.” Adding drawdown areas into the emissions equation, dams release twice as much carbon globally on average than they store.

But here the science gets complicated and murky: On the flip side, Koschorrek says, drawdown areas also seem to emit less methane. “If the water level goes down in reservoirs and we have these dry areas, then we increase the CO₂ emission from the whole system, but at the same time, we reduce the methane emission from the water surface.”

More research will be needed to determine how these emissions, or lack thereof, sort themselves out, weighing a multitude of factors, such as tropical vs. temperate location, types of vegetation involved, and more.

Can we live with dams?

Humanity’s future relationship with dams will likely remain an uncomfortable, ambivalent partnership.

“We’re not an anti-dam organization; we recognize the value and benefits those dams bring to society,” says WWF’s Thieme. “The holistic view in the long run, I think, is one that will allow for a greater ability of the [freshwater] system to be resilient in the face of a changing climate. In practice, we will have some parts of our rivers that are more working rivers and some parts we keep free flowing. That’s the ideal because, to survive and to flourish, we also need to use water in ways that are sustainable.”

Put simply, we need to learn to live with some dams, while scientifically balancing their benefits against their harms.

An example of this attempted balance can be found in the Penobscot River Restoration Project. This effort to revive New England’s second-largest river system entailed the removal of two dams and construction of a stream-like bypass channel around a third. Hydropower generation was increased at six nearby dams to compensate for the removed dams. The project has given locally endangered Atlantic and shortnose sturgeon and striped bass unobstructed river access to their historical habitats, opening 3,200 km (2,000 mi) of river and tributary habitat for sea-run fish.

Back in Kruger, Midzi reckons there’s enough natural water in the landscape for animals to survive and maintain ecological function. “People do not need to panic when a dam is being removed. It doesn’t mean there is any less water for the wildlife.” Even in dry seasons, natural surface water is available in pans, springs and pools, or below the sand, which digging animals such as elephants can access. During severe drought years, most wild animals are affected more by lack of food rather than water, Midzi says. But by removing artificial infrastructure, he says, park managers are reversing past mistakes.

The reality is that Kruger, like other parts of the world, will probably remain at least somewhat dependent on dams into the future, even as officials continue removing dams inside the national park. Riddell notes that water releases from South African dams located upstream, outside the park, maintain water flows that benefit the environment inside the preserve while also aiding various industries and farmers.

In the end, dams are a mixed blessing, Riddell says. “From an ecological point of view, you want to oppose that dam, but you also realize you kind of need it.”

This article was originally published on Mongabay