For years, geologists and geographers from Brazil and abroad have been studying the landscapes that shape the floodplain of the Rio Negro, in the southern portion of the Pantanal. Even after all this time, the experts were unaware that under a huge carpet of aquatic plants, there was a huge hidden lake.
In a paper published in October in the scientific journal Earth Surface Processes and Landforms, researchers from São Paulo universities described the discovery of a lake that occupies an area equivalent to a little over two-thirds of the city of São Paulo. “It was almost impossible to identify it at first sight,” said geographer Eder Renato Merino, a researcher at the Institute of Energy and Environment of the University of São Paulo (IEE-USP), one of the authors of the study.
“From a distance, the accumulation of water hyacinths and baceiros along its surface makes it look like an area of land covered by grass.” Water hyacinths—floating aquatic plants of the genus Eichhornia—and baceiros are islands of floating vegetation in the middle of flooded areas.
Mr. Merino is one of those credited with identifying the lake, located in the floodplain of the Rio Negro. The finding was the result of satellite imagery analyses carried out during his Ph.D. course, concluded in 2016 under the guidance of geologist Mario Luis Assine, of the Institute of Geosciences and Exact Sciences of the São Paulo State University (Unesp) in Rio Claro. For decades, he has been studying the transformations that the landscape of the Pantanal—one of the world’s largest floodplains—has undergone. In this case, he and Mr. Merino were interested in investigating the formation of the Rio Negro plains and the process of capturing and distributing water and sediments in the region during the rainy season, between the months of October and May.
Uncovering the lake
During analysis of the course of the Rio Negro, from its sources on the plateau to the lower and flatter lands of the Pantanal, they observed that this body of water spread as it advanced over the floodplain. It changed course and created several winding canals along the way. So far, nothing new; the changes in the trajectory of the rivers in the Pantanal are well known by researchers. The surprise came when they realized that the channels seemed to disappear in a certain region of the map, which was apparently composed of land. It was as if the canals were diving underground to resurface further ahead. “It was a deserted and totally flat area, with no buildings, roads or signs of cattle raising,” says Mr. Assine.
To understand the origin of the phenomenon, Messrs. Assine and Merino looked at a historical series of remote sensor images from the region, obtained by the Landsat satellite between 2000 and 2011. “We analyzed the images and then flew over the site,” says Mr. Merino. “We found that there was a lake, hidden under a huge blanket of water plants,” he explains. Back at the laboratory, the researchers made more detailed analyses of the images.
The objective was to determine the frequency of flooding and the humidity index in that area. Using computational models, they found that there was a permanently flooded area of almost 1,100 square kilometers, even in the seasonal dry periods between May and September. They concluded that this was a large lake, which had not been mentioned until then.
The lake is estimated to be between three and four meters deep, with some variation in water level between the dry and rainy periods. Its bed is composed of—among other materials—a thick layer of fine sand and mud enriched by organic matter formed from decomposing plants. According to Mr. Merino, it is supplied by the waters of the Negro and Aquidauana rivers, especially during the rainy season. Compared to the Rio Negro, the Aquidauana river is mainly responsible for transporting sediments into the lake. This is because, unlike the Aquidauana, most of the sediments of the Rio Negro are trapped in higher lands before descending to the floodplain.
Peculiarities of the findings
The behavior of the two rivers that feed into the lake is also completely different from one another. The waters of the Rio Negro run through several channels, which form at the bottom of the lake. Meanwhile, the waters of the Rio Aquidauana cross the flooded area by a single surface channel. According to Mr. Assine, this channel was formed during the high season. At that time, he explains, the waters of the river, rich in sediments, overflow into the lake. “The sediment that overflows with the water accumulates on the lateral margins of the Rio Aquidauana, forming elevated lateral dikes, which form a channel through which the river passes when its water levels decrease again,” he explained.
The researchers also analyzed the plants that cover much of the lake’s surface. According to them, the vegetation has long roots that spread below the waterline. Transported by rivers, these plants accumulate along the surface and, in many cases, form islands of vegetation in the middle of the flooded area. Some of these islands are being colonized by pioneer plant species that are able to settle and multiply rapidly in inhospitable environments.
“This is the first time that this type of fluvial lake has been discovered in the Pantanal,” says Mr. Merino. It is estimated that the immense body of water was formed from movements of tectonic plates, which are still frequent in the region. The associated geological faults produce earthquakes with a magnitude of around 3.5 degrees on the Richter scale and are probably responsible for the progressive lowering of the area in the middle of the Rio Negro floodplain, favoring the accumulation of water and the formation of a large river lake. “This is a very well-preserved region that can only be accessed by boat,” says Mr. Merino. “The area is also very rich in biodiversity, offering favorable conditions for the reproduction of several species of birds, reptiles, and mammals,” adds Mr. Assine.
The biodiversity of the Pantanal
Geographer Renato Lada Guerreiro, professor at the Federal Institute of Education, Science, and Technology of Paraná, highlights that the Pantanal is a complex environment, comprising a vast biodiversity of species and a wide variety of ecosystems. “Identifying all these ecosystems in detail is a very complex task, whether by means of satellite images or field expeditions,” said the researcher, who did not participate in the study. “The work of Messrs. Merino and Assine is an example of how we can deepen our knowledge of the landscapes that shape this biome.”
According to Mr. Guerreiro, the study also makes it even more evident that the systems formed by lakes in the Pantanal are not the results of the same processes. “The origin of this ‘hidden lake’ in the Rio Negro floodplain resulted from geological, hydrological and climatic processes that are completely different from those that gave rise to the Nhecolândia lagoons, near the municipality of Corumbá in Mato Grosso do Sul, even though the two areas are very close to one another,” says the researcher. The main difference between lakes and lagoons is their size. Though both are defined as an area of water surrounded by land, lakes, in general, are larger than lagoons.
According to Guerreiro, the lagoons of Nhecolândia are said to have emerged from depressions shaped by the accumulation of fine sand grains carried by the wind in dry periods at the end of the Pleistocene, between 20,000 and 15,000 years ago. “To study the Pantanal is to be surprised with new discoveries at every moment,” said Mr. Assine.
Cited in this article
1. Mudanças paleo-hidrológicas, cronologia de eventos e dinâmica sedimentar no quaternário da bacia do Pantanal (nº 14/06889-2); Modalidade Auxílio à Pesquisa – Regular; Pesquisador responsável Mario Luiz Assine (Unesp); Investimento R$ 253.715,39.
2. Mapeamento das lagoas salinas e cristalinas do Pantanal de Nhecolândia por meio de sensoriamento remoto ótico e interferométrico (nº 17/26318-8); Modalidade Bolsa de Pós-doutorado; Pesquisador responsável Adolpho José Melfi (USP); Bolsista Eder Renato Merino; Investimento R$ 209.304,09.
Merino, E. R. e Assine, M. L. Hidden in plain sight: How finding a lake in the Brazilian Pantanal improves understanding of wetland hydrogeomorphology. Earth Surface Processes and Landforms. out. 2019.