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The Amazon is dazzlingly diverse and safeguarding it is central to global efforts to tackle the climate and nature crises. But when it comes to the threats faced by the Amazon, we often focus solely on deforestation and forget that the river and its tributaries - the largest river system in the world by far - are also at real risk.

This Amazon Day we must recognize the serious threats to Amazonian rivers, particularly planned hydropower dams, such as those looming over the Tapajós River in Brazil - dams that could fragment free flowing rivers and destroy the diverse benefits they provide to people and nature.

The 2,000 km long Tapajós River in Brazil is one of the largest, free flowing tributaries of the Amazon, accounting for around 6 percent of the water in the entire basin. Unusually for the region, it is also a fast-flowing, clearwater river, which sustains diverse communities as well as extraordinarily rich biodiversity.

Flowing through habitats ranging from cerrado savanna to rainforests, the Tapajós boasts a treasure trove of species, including at least 325 freshwater fishes - 65 of which are endemic to the river. But scientists believe that many more are still waiting to be discovered, estimating that there are more than 500 fish species swimming in the Tapajos basin. Thousands of other plant and animal species also rely on the river, including many iconic, threatened species, such as river dolphins, giant otters, giant anteaters and jaguars.

Indigenous people and local communities depend on the Tapajos to support their fisheries, fertilize their fields, provide clean water, and sustain their livelihoods and cultures. Natural flows and flood pulses are critical to life along the river, where conditions can fluctuate widely in the wet and dry seasons.

Unfortunately, all this is under threat. Like the other iconic rivers around the world highligthed in WWF's new 10 Rivers at Risk report, the Tapajós River could soon be dammed. The last of the large free flowing tributaries on the right bank of the Amazon River, the Tapajos is regarded as one of the best remaining opportunities for hydropower generation. A total of 42 dams are currently planned in the basin, including a series of five barrages comprising the huge Tapajós River Hydroelectric Complex.

If all these dams were built, the environmental and social consequences would be disastrous. An estimated 2,000km2 of Indigenous territories would be flooded by reservoirs. The dams would alter the river’s natural water flow, depth, temperature, sedimentation and oxygen levels, and would destroy the delicate ecosystems - and threaten the wealth of wildlife - that the river currently supports. The rapids and waterfalls that characterize the Tapajós would be destroyed, fisheries would disappear as migration routes were cut, and floodplain fertility would fall.

What’s more, the hydropower dams themselves are based on out-dated forecasts - and 20th century thinking - which do not take into account increasing energy efficiency nor the fact that there are now better alternatives due to the plunging price of wind and solar generation and battery storage technologies. Nor do they properly factor in the impact on communities and wildlife or the broader, knock-on consequences of the loss of all the diverse benefits that a healthy, free flowing Tapajos provides to people and nature.

And to efforts to tackle the climate crisis. One study shows that the carbon and methane emissions from the construction of the dams and from their reservoirs would generate greenhouse gas emissions comparable to a natural gas plant, with the worst of the five dams - at Cachoeira do Caí - producing total emissions that would exceed a coal-fired power station.

Nevertheless, despite the pervasive influence of Brazilian construction lobby and the ardently pro-dam position of the Bolsanaro government, opposition to the dams has been – and continues to be – immense. The indigenous Munduruku tribe, in particular, has fought tirelessly to protect their ancient lands, and court battles have been raging for years. There were encouraging signs in 2016 when the environmental licence for the largest proposed dam at São Luiz do Tapajós was officially cancelled, but dams often come back from the dead. And there are still many more on the drawing board.

But this is not a battle that the world can leave to Brazil. If the hydropower complex on the Tapajós goes ahead, the immediate impact will be felt by indigenous people and local communities - and nature - in the river basin. But ultimately the effects will be felt all over the world: this is a looming tragedy that should concern the entire global community.