In a remote corner of Venezuela, a hydrographic rarity occurs: the Orinoco River delivers a quarter of its flow to the Negro River and from there to the Amazon through the Casiquiare Branch. This unique connection between the two great rivers of the continent, a point of intense biodiversity and a critical bridge to the jungle biome, is wounded by the advance of deforestation, illegal mining and climate change.

By: Reybert Carrillo (El Estímulo)

At the end of 2023, Brazilian scientists found more than one hundred Amazon river dolphin  (Inia geoffrensis) in Lake Tefé, in the state of Amazonas, some 600 km from the city of Manaus. The tragedy alarmed Venezuela because it occurred near the Brazilian border.

The Venezuelan State sent a technical commission to San Carlos de Río Negro, the last town in the south of the country, to investigate the causes. I was on that commission.

As I traveled aboard a small plane flying through the Amazonian sky, I thought about how I had always wanted to understand the world’s largest water robbery, the one that the Amazon River made to the Orinoco, through the Casiquiare Arm. I had been waiting for that opportunity since I was a geography student at the Universidad de Los Andes, in Mérida. It was the moment.

We landed in San Carlos de Rio Negro on November 17, 2023. The objective was to study climate anomalies in the Amazon. Video: Reybert Carrillo

The Casiquiare is the only proven connection between the two basins. It is an aquatic corridor that detaches from the Orinoco and takes with it a quarter of its flow. These unruly waters meander southward and join the Negro River, the main tributary of the Amazon River, consummating a truly unique form of hydrographic theft on the planet.

The Casiquiare Arm is the only aquatic link between the Orinoco and the Amazon. Cartography: VE360 non-profit organization.

I saw it for the first time at dusk on November 17, 2023 from the Cessna plane in which I was flying. The penillanura (a gently rolling plain) that escorted it downstream also seemed complicit. I scanned it through the window in a quick pan before landing at the small airport of San Carlos de Rio Negro and paid my respects. The Casiquiare Arm returned a greeting in the Baré language and invited me in.

The Casiquiare Arm as seen from the air, November 2023. Video: Reybert Carrillo

After several days of visiting the site, the institutional mission concluded that the death of those dolphins in Brazil was due to the chemical alteration of the lake where they lived, caused by the increase in temperatures and the drought affecting the entire region.

But my personal objective was just beginning. Seeing from the sky the deforested patches, the mining pockets, and the sepia tone that made the air rarefied, made me suspect that the Casiquiare basin could be threatened.

A flight over the Casiquiare that raised concerns, November 2023. Video: Reybert Carrillo

I decided to investigate the matter and, two years after that overflight, I was able to confirm my suspicions thanks to a collaboration between Latin American journalists and scientists, promoted by the Serrapilheira Institute of Brazil and the Latin American Center for Investigative Journalism (CLIP) to explore how damage to the Amazon’s biodiversity disrupts the different environmental services it provides to the continent. 

Understanding Casiquiare from history

The behavior of this basin is unprecedented; it has been discussed for centuries in America and Europe, in the Cocuy stone -with the ancestral language of the Yanomami- and in university classrooms.

Jaime García, a native of San Carlos de Río Negro and member of the Baré ethnic group, an indigenous people settled in the Casiquiare basin, also confirmed this to me. I met him on that trip and contacted him again two years later:

“This watery arm, as the whites call it, is a source of sustenance and purification. It is the first bath in the morning and the blessing of the fish at our table. It is the way to go to the villages. In the Casiquiare we set the words of our language in motion, so that they may reach hidden places and never run out.”

The connection of the Casiquiare with the other rivers was already known by the ancestral peoples, but the first non-native groups to navigate it were the Portuguese conquistadors. One of the people who knows this history well is the Venezuelan explorer and naturalist Charles Brewer Carías.

I interview him at his home in Caracas, surrounded by books and old maps. He tells me about Manuel Román, a Jesuit missionary known for being the first Spaniard to navigate the Casiquiare in the 18th century and also a pioneer in documenting the connection between the Orinoco and Amazon rivers.

Charles Brewer Carías recalls and paraphrases Jesuit Father Manuel Román in his interaction with Portuguese merchants about the connection between the Orinoco and the Amazon through the Casiquiare. February 2025. Video: Reybert Carrillo

Half a century after Father Román’s feat, it would be the German naturalist Alexander von Humboldt who would offer the greatest contributions on this basin.

“For half a century no one has doubted the existing communication between the two great water systems, but our mission was to fix by astronomical observations the course of the Casiquiare,” wrote Humboldt in his well-known and widely circulated book ‘Voyage to the Equinoctial Regions of the New Continent’ (1826).

In addition to mapping the rivers, Humboldt navigated the basin and documented different species, together with the French botanist Aimé Bonpland. He had the impression that he was in a very special place on this planet, a corner of Venezuela where nature should be preserved. 

Brewer Carías understood the same thing a century and a half later, when he began to explore the area. He has traveled the Casiquiare basin and knows the entire Upper Orinoco. That is why today he is one of the most influential Venezuelan voices when talking about preserving these forests. 

In early 1990, Venezuelan President Carlos Andrés Pérez called on Brewer Carías to put his knowledge of the Upper Orinoco and Casiquiare at the service of the Republic. The Venezuelan explorer, who had already been Minister of Youth between 1979 and 1982, answered the call, together with the American anthropologist Napoleon Chagnon. Both consolidated a detailed report on the importance of creating a Biosphere Reserve in that portion of the Venezuelan Amazon.

“The garimpeiros (artisanal gold miners) were a threat to the indigenous communities of the Casiquiare and Upper Orinoco. So was malaria. We knew the territory, we had been in the Siapa and we saw the situation of the Yanomami up close,” Brewer Carías told me.

He recalls in detail that the Yanomami had hardly any contact with the outside world, but both garimpeiros and missionaries were interfering and disturbing their ancestral worldview.

“The biosphere reserve would help keep these communities pristine. We explained it to the president and he wisely listened to us,” says the explorer.

On June 5, 1991, the Pérez government decreed the creation of the Alto Orinoco-Casiquiare Biosphere Reserve, which would be recognized by UNESCO two years later, on October 9, 1993, as the first in Venezuela.

A Biosphere Reserve is a territory that seeks to make environmental preservation compatible with the sustainable use of resources. They are proposed by states and validated internationally by UNESCO.

Venezuela’s first biosphere reserve. Cartography: VE360

National parks and natural monuments in the reserve. Cartography: VE360

The Casiquiare would allow Venezuela to promote before UNESCO the first Amazonian RAMSAR site in national territory. This designation is granted to wetlands and bodies of water that safeguard biodiversity or have a rare hydrographic structure.

The Orinoco-Casiquiare-Negro-Amazonas connection determines one of the most important biogeographical links on the planet. Video: Reybert Carrillo

The Casiquiare basin, a hydrographic rarity

The Casiquiare basin mapped by VE360 with the benefits and privileges of contemporary technology.

The one that Humboldt mapped in 1800 -and I observed from a light aircraft in 2023- is a basin of 44,160 square kilometers, whose course extends for 326 kilometers, crossing an ecosystem that is both riparian forest and tropical rainforest.

It receives several tributaries: the Pasimoni, the Siapa, the Pasiva and the Pamoni; it then flows into the Negro River, the main tributary of the Amazon.

Measurement of the flow and depth of the Negro River after receiving the waters of the Casiquiare. San Carlos de Río Negro, November 2023. Video: Reybert Carrillo

Most rivers originate in wetlands or as rain catchments in mountainous headwaters. The water descends and forms threads of water that scratch and erode the watershed. But the Casiquiare is different. It does not originate from a mountain, but from another river. From one of the largest: the Orinoco. It escapes and flows downstream until it ends up in another even longer and more abundant river, the immense Amazon.

Explaining it has been complex for specialists from different places and times. Among the Venezuelans, Gustavo Silva León, a geographer from the Universidad de Los Andes, stands out. He has been studying the Orinoco in all its extension for forty years, including the Casiquiare. That is why I turn to him to understand the hydrological components of this intrepid river.

Silva explains to me that its aquatic capture on the Orinoco is curious, since it is a common behavior in the rivers of the plains, but not of the Amazon, and that, when a river steals the waters of another, it usually does so totally, not partially. But Silva’s studies indicate that the Casiquiare steals about 350 thousand liters of water per second from the Orinoco. That is, a quarter of its flow. Another striking element for the geographer is that such a rarity occurs in a peneplain and not in a plain.

“In the floodplains of Venezuela it is common to see branches escaping from one river to join another. It is also usual to see rivers changing trajectories and leaving only the scar of the old riverbed. The Casiquiare is the curious exception to that rule, as it occurs in an environment that is not completely flat,” Silva explains to me as he traces with his finger the meandering path of the Casiquiare on the map without looking at it.

A biodiverse hotspot

The richness of the Casiquiare is not only due to its strange hydrographic behavior, but also to its biodiversity. Humboldt described it extensively two centuries ago, which helped to understand this region as what biologists call a biodiversity hotspot. That is, a place of concentration of many living species.

More recent studies confirm this. In 2018, the Polar Foundation claimed that the ichthyofauna (fish) of the Casiquiare was one of the richest and least documented on the planet, an opinion based on the 88 species of ornamentally important fish found in the basin.

In a study on fish consumption patterns in the area by FAO, Venezuelan ichthyologist Carlos Lasso characterized 25 mammal, 80 bird and 6 reptile species in the area. Birdlife International classified Casiquiare as an ‘Endemic Bird Area‘ in 2014. And an expedition of Venezuelan and Colombian ornithologists counted 234 bird species in the basin in 2023 for the ornithological platform eBird, of which eleven were sighted in the basin for the first time.

The Casiquiare river dolphins

The biogeographic bridge of the Casiquiare plays an important role: it favors emblematic species that are threatened, such as river dolphins, also known as pink dolphins, like the ones that had died in Lake Tefé. Although during my brief visit to the basin in 2023 I did not have the chance to see dolphins, I interviewed someone who did.

Yurasi Briceño is a biologist at the University of Zulia and directs the Sotalia Project, an environmental organization that protects cetaceans in Venezuela. In 2023, Briceño participated in the Casiquiare Expedition, along with 15 other scientists and four journalists, and observed 387 dolphins in the basin for about three weeks.

The biologist tells me that the logbook came out of the South American River Dolphin Initiative (SARDI), a conservation team that brings together the efforts of the six Amazonian countries where the species lives.

The expedition was led by Colombian biologist Fernando Trujillo, director of the Omacha Foundation and recognized worldwide for his work on these cetaceans and the rivers that shelter them. His most ambitious objective was to make a population assessment of the species in the Casiquiare.

“The number of dolphins we observed was higher than the census conducted ten years ago. This does not mean that the population increased, but that we applied a more efficient methodology and more sophisticated technology”, Briceño tells me and adds that this expedition is a pioneer in the use of remote sensors to track this species in Venezuela.

These hydrographic conditions have been changing in recent years. According to the Latin American scientific geoportal MapBiomas, a total of 1,020 hectares covered by water in 1985 became forest in 2023. Another 3,387 hectares that were forest in 1985 became water bodies in 2023.

This means that in the basin the opposite is happening to the regions where drought is disappearing rivers by the droves. But this change has not been favorable, on the contrary.

The impact is felt by biodiversity, as many species are susceptible to climatic anomalies, including dolphins.

Briceño explains to me with concern the implications: “In times of drought, the dolphins move along the main channel, but when there is a lot of water in the basin, as a result of increased rainfall, they look for other channels that they do not usually travel”.

The increase in cases of stranded dolphins in recent years, not only in the Casiquiare, but also in the entire Orinoco region, is due to the alteration of water flows.

“River dolphins know how to differentiate rainy seasons from droughts. Their organism is adapted for that. If it rains when there should be a drought, the dolphins get out of control and cannot distinguish the streams with enough water. Then they get stranded and die,” adds Briceño. The death of these dolphins and those of the Tefé are part of the same problem.

The impacts of climate change 

After consulting with sources such as García, Brewer Carías, Silva and Briceño, who helped me understand different aspects of the basin and the peneplain, I went back to review the timeline in MapBiomas and found another key piece of information: MapBiomas ranks the Casiquiare as the seventh Venezuelan basin with the most changes between 1996 and 2023. During this time period it gained 2,507 hectares of aquatic space. The reason? The alteration in the frequency and intensity of rainfall caused by climate change.

The data indicated that, starting in 2008, there have been peaks and troughs in the basin’s flows, i.e., times when a lot of water flows and times when it decreases drastically. In 2010, for example, there were 35,664 hectares of aquatic space. Just two years later, that figure increased to 46,290 hectares.

I found the revelation disturbing. Perhaps it would be worthwhile to contrast it and complement the analysis with another database that I had obtained during my time as a public servant.

These data had gaps due to weaknesses in the network of weather stations that captured them. So scientists Luis Cattelan and Maria Luiza Busato, from the Federal University of Santa Catarina and the State University of Campinas respectively, now in the quantitative ecology training program at the Serrapilheira Institute, created a simulation model that reconstructed the missing data.

The reconstruction allowed me to identify a set of climatic anomalies. For example, in 2023 it rained in a period that should have been dry; just one year later, in 2024, droughts occurred during historically rainy periods.

The contribution of the Brazilian scientists finally validated the hypothesis that was born out of my concerns when I was flying the skies over the Casiquiare: climate change was hitting the basin.

Deforestation and mining also hit forests

The image that had remained in my memory of the overflight included several patches of logged forest. They were forests that had never been logged, those that scientists call “primary forests”. How far had deforestation advanced over primary forests?

The Global Forest Watch platform states that in the two decades between 2002 and 2023, at least 3,070 hectares of primary forest will be deforested in the Casiquiare-Río Negro junction.

The importance of Amazon trees is that they are containers of carbon dioxide (CO2) . But when they are cut down, all that CO2 goes into the atmosphere and alters natural cycles on a planetary scale.

According to Global Forest Watch, deforestation in the Casiquiare-Rio Negro junction between 2001 and 2023 emitted 2,870,000 tons of CO2 equivalent, a figure that is derisory compared to the 300 million tons that the Amazon biome as a whole emits per year, but which does not detract from the seriousness of the issue.

One of the causes of deforestation in this area is illegal mining.

I would have liked to see it with my own eyes, but due to the impossibility of traveling again, I spoke with someone who was in San Carlos de Río Negro in 2024. This person also visited the vicinity of Cerro Yapacana National Park, located 150 kilometers from Casiquiare. There he witnessed the mining escalation. He gave me his testimony on the condition that I keep his identity confidential to avoid reprisals from the Venezuelan government and criminal groups operating in the area.

“On that trip I collected testimonies from people who have done mining in the Yapacana. It is an easy way to make a living. The Yapacana is the cash register of the state of Amazonas. Those who are in need spend some time there,” he tells me.

The source discovered that, on the top of the tepui, there is an implausible infrastructure of bars, discotheques and billiards, incompatible with the environmental dynamics and the original ancestral cosmogony.

“In the memory of the villagers is fresh the ‘Operation Autana‘ [military incursion that evicted illegal miners in December 2022]. They remember it as something violent and traumatic. What I perceive is that most of those who have been mining in Yapacana are not criminals. Those who run the mine are, but the common miner is looking for a livelihood,” he explains.

Jaime García, the Baré member I met in San Carlos de Río Negro, assured me that, as in Yapacana, there is also mining in Casiquiare.

“There is mining on the banks of the Guainía, the Siapa and the Pasimoni. It is visible, it is not something new. It has been going on for more than forty years, and although it is not as aggressive as in Las Claritas or El Dorado [Bolívar state], it also degrades our Casiquiare,” García adds.

Your people are losing their children. Mining is environmental destruction, but it is also the loss of values of the Baré and other indigenous peoples. Many decided to go to the mine and succumbed to drugs, alcohol and prostitution.

A work of the Venezuelan portal ArmandoInfo, published in 2022, identified 42 clandestine airstrips in the 3,718 mining points between Bolivar and Amazonas. One of them was near the Siapa-Casiquiare confluence.

Motivated by ArmandoInfo’s finding, I searched for recent satellite images from Copernicus, the European Union’s aerospace program. I found the Siapa track and saw it active, at least, until March 4, 2024. In another image from August 18 of that year the track is no longer clearly distinguishable, so it is possible that it has been deactivated. Images from September (2024), March and April (2025) also do not show the track.

Colombian criminal groups calling themselves dissidents of the Revolutionary Armed Forces of Colombia (FARC), a historic guerrilla group demobilized in 2016, and garimpeiros [Brazilian miners] are present in Amazonas’ Rio Negro municipality, according to information provided by Venezuelan NGO SOS Orinoco. These gangs use clandestine tracks and they are present in the Siapa, San Miguel, Pedaraguén, Pimuchín, Irene, Daniacushím, Denarikén, Lapa, Vanisa and Amariquén rivers.

Mining in the Casiquiare watershed. Cartography: VE360 with information from SOS Orinoco.

It is men – the garimpeiros, the Colombian criminal groups, the deforesters using fossil fuels – who are threatening the Casiquiare. 

Thus, any strategy for the preservation of this watershed must be understood as a complex fact that requires systematic solutions, and none of them will work in isolation. Attacking mining implies responding to deforestation, and vice versa. Conserving the river dolphins means involving indigenous peoples in the process. These solutions must be simultaneous and complementary; the most effective strategy for this is the environmental nursery, but this requires large doses of political will that Venezuela does not have today. 

“The purpose of our mission was to fix Casiquiare astronomically. That objective would be frustrated if we lacked the sun and stars,” said Humboldt in 1800. Today’s Venezuela means that much more than astronomy is needed to navigate through the Casiquiare as Humboldt did, but one can still summon the echo of his voice to keep alive the desire to do so.

Sunset on the banks of the Black River in November 2023. Video: Reybert Carrillo