Kiyonga was keen to explain. “We have six different types of mangroves here,” she said. Different species were used by villagers to build their houses and kindle fires, to make fences, beehives, fishing floats, and poles for pounding grain, as well as to manufacture dyes, medicines, and alcohol from fermented sap. ­­Meanwhile, their roots in the silty waters harbored breeding fish and prevented erosion by the ocean waves.

“They are increasing because we are taking better care of them,” she told me. “We have areas where we harvest, areas where we don’t, and area where we plant”

But their stewardship could soon be in vain, for the month after my visit, the Tanzanian government confirmed plans to build Africa’s second-largest hydroelectric dam in a gorge 100 miles upstream on the Rufiji River, signing a contract with an Egyptian state-owned engineering company, Arab Contractors. It is set for completion later this year.

Environmental scientists warn that the $3.6 billion dam will have a devastating impact on the delta — all but ending the river’s wet-season floods, which bring in freshwater and silt. The mangroves will die; the fish will disappear; and the delta itself will start to be eroded by the ocean. Coastal villages such as Jaja will be the first to go. But the people of Jaja and others across the delta that I visited had never heard of the dam plan, much less been consulted.

The Stiegler’s Gorge dam may become one of the most environmentally damaging hydroelectric projects ever constructed in Africa.

The project directly threatens two large protected areas. The Selous Game Reserve, in which the dam is being constructed and its reservoir is situated, is a UNESCO World Heritage Site almost twice the size of Massachusetts and widely regarded as one of Africa’s finest, largest, and most pristine wilderness areas. And the Rufiji delta is part of a larger coastal wetland area known as the Rufiji-Mafia-Kilma Seascape, recognized for its international importance under the Ramsar Convention.

“It is unprecedented to risk losing the integrity of not one, but two globally significant protected areas to a hydropower project,” writes Joerg Hartmann, a Colorado-based water and energy consultant familiar with the project.

But with local activists facing threats of jail if they criticize the dam, few people outside Tanzania have heard about the unfolding calamity.

The concrete-and-rock dam, 430 feet high and 2,300 feet wide, barricades the Rufiji River as it leaves the five-mile long Stiegler’s Gorge and enters a 100-mile-long floodplain of shifting riverbeds, marshes, lakes, and mangroves that leads to the Indian Ocean.

With an installed capacity of 2,100 megawatts, the dam will be as powerful as Egypt’s High Aswan Dam on the Nile and exceeded in Africa only by the recently completed Grand Renaissance Dam in Ethiopia. It will hold back a reservoir 62 miles long and more than seven miles wide on average.

Critics say that while the reservoir will only directly inundate around 2 percent of the giant Selous reserve, it will obliterate key wetlands and block migration routes used by many of its 750,000 large mammals, which belong to 57 species, including elephants, black rhinos, cheetahs, buffaloes, crocodiles, giraffes, and hippos. Meanwhile, the buildout of roads, power lines, construction camps, and other infrastructure needed for the project is already leading to an upsurge in poaching and extensive deforestation. In preparation for construction, the Tanzanian Forest service contracted the clearance of 2.6 million trees from 570 square miles of forest.

As construction work got under way in 2019, officials at UNESCO recommended that the Selous reserve be delisted as a World Heritage Site, because of the dam. But at a meeting of the organization’s World Heritage Committee last June, the Tanzanian government rallied fellow African governments to veto the delisting, at least for now.

Concern about the fate of Selous has distracted attention from what experts say is arguably an even greater threat to the floodplain and delta ecosystems downstream, on which some 200,000 people depend for their livelihoods.

Despite two smaller dams on tributaries, the Rufiji is “essentially the last major relatively free-flowing river in East Africa,” according to Barnaby Dye of the University of Manchester. When completed, the dam will end that. It will be able to hold back around a year’s river flow. And if operated to maximize hydroelectric power generation — which Dye says appears to be the intention of its state-owned operator, the Tanzania Electric Supply Company (TANESCO) — it will largely halt the wet-season flood of water and sediment that maintains lakes rich in fish, as well as fertilizing fields and maintaining the mangrove-covered delta.

“Floodplain lakes will no longer be connected to the river and [will] dry out, and the delta and beaches will be subject to erosion,” writes Hartmann. Salty seawater will invade previous freshwater areas of the delta. Fields nourished by annual flooding from the river will lose silt and nutrients, he adds.

The World Wildlife Fund (WWF) says the dam will trap most of the estimated 16.6 million tons of sediment that flow down the river each year. Without that silt supply, “the river bed will deepen, river banks will collapse,” warns Hartmann. And the delta will be eroded away by the ocean, says Kjell Havnevik, a development researcher at the University of Agder in Norway, who names coastline delta villages Jaja, Mbwera, and Pombwe as at particular risk of being washed away.

In theory the government should be alerted to such risks through its legally required environmental impacts assessment (EIA), published in 2018, and a strategic environmental assessment (SEA) completed the following year. But both government-commissioned reports have been widely condemned as vague, biased, technically inadequate, and predicated on the assumption that the project must go ahead regardless.

The EIA claims, for instance, that water releases from the dam would be sufficient “to keep the floodplain lakes connected to the Rufiji River for most of the months of the year and thus sustain floodplain fish production,” and would “alter the geochemistry of the delta in favor of maintaining the optimum balance of salinity regime to ensure ecological integrity of the delta.”

But neither claim was backed up with evidence, and Yale Environment 360 could not find independent experts who agreed with them.

Hartmann, who has drawn up environmental guidelines for the International Hydropower Association and who worked in Tanzania for several years, says the EIA shows “no serious technical assessment of downstream impacts,” which he says was “an irresponsible and unforgivable mistake. Compared to international good practice in hydropower, this is an unacceptably superficial level of information.”

A review of the SEA for the International Union for the Conservation of Nature (IUCN) found that it “falls far short of normal standards … for projects of this magnitude and complexity” and “does not provide a sound basis for decision-making.” A claim in the EIA that the dam would bring benefits for downstream communities by reducing dangerous floods “is not based on credible reasoning or evidence,” according to a separate IUCN-backed review. The IUCN has called on the government to “permanently abandon” the project.

None of this criticism should have come as a surprise to the Tanzanian government, least of all to the author of the EIA, Raphael Mwalyosi of the Institute of Resource Assessment at the University of Dar es Salaam. Back in 1988, as a young biologist, he warned that “the most significant effect” of building a dam in Stiegler’s gorge would be a “drastic reduction” in flood flows downstream. “Floodplain fisheries would totally collapse,” he wrote. “Some mangrove stands in the delta would probably be displaced by reeds.”

Mwalyosi did not respond to emailed requests for comment on whether, or why, his view had changed.

Government officials knew too. When Tanzania successfully nominated the Rufiji delta as part of the Rufiji-Mafia-Kilma Ramsar site in 2004, its Ministry of Natural Resources noted that a dam at Stiegler’s Gorge would “have severe impacts on the ecological balance downstream,” damaging biodiversity, fisheries, and livelihoods.

But the urge to build the dam — first proposed by German colonial engineer Franz Stiegler more than a century ago — has refused to die. It became a cause celebre of President John Magufuli after his inauguration in 2015. He saw it as a vital source of electricity to propel his nation’s economic advance. He rejected those, such as Hartmann, who noted the country already suffers power outages when droughts hobble two other hydro plants in the Rufiji basin and explained that Tanzania has alternatives, including “excellent solar and wind potential, located close to load center and transmission infrastructure.”

To help mitigate downstream impacts, hydrologists have said that any dam on the Rufiji should be operated to allow releases of water and silt during the wet season, proposing a flow of 2,500 cubic meters per second.

But Dye argues that the proposed seasonal release is too meager and that “the majority of downstream lakes will be cut off and would therefore dry up or lose their ability to sustain fish.” Stephanie Duvail of the Institute of Research for Development in Marseilles, France concludes there would be “a huge impact on the livelihoods of the Rufiji floodplain inhabitants, potentially pushing many people to the brink of survival.” She found that without the river flooding farmland, crop yields would drop in half within three years.

TANESCO did not respond to requests for comment on either the potential downstream impacts of its dam or the possibility of ensuring wet season releases of water and silt.

Rather than engaging with such concerns or looking for less damaging sources of power, Magufili’s government has sought to shut down debate. In mid-2018, environmental minister Kangi Lugola warned that anyone who resisted the project would be jailed.

The warnings were heeded. One NGO leader in the country told Yale e360, “This is a highly sensitive topic, and most NGOs did not want to get involved further, despite opposing the dam.” After making early criticisms of the project, WWF International largely ceased making statements in 2019, and a spokesman confirmed that its national office in Tanzania has kept entirely silent.

Magufuli died in March last year, but his successor, Samia Suluhu Hassan, has pushed on with the project.

When asked if, with the dam almost built, now would be the time to campaign for maximum downstream flows, officials at two leading environmental NGOs working in the country declined to involve themselves, for fear of offending the government.

So, by the end of this year, it seems likely that the completed dam will begin filling Steigler’s Gorge with water. The downstream ecosystems and the people who depend on them will await their fate.