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Geology

Invading nomad

The Taquari River changes course and permanently inundates an area of 6 thousand square kilometers in the Pantanal

Until the end of last decade, Aguapé farm, a 15-thousand hectare property dedicated to cattle raising located in the region of Paiaguás, in the north of the state of Mato Grosso do Sul, was in peace with the regime of floods and droughts that characterizes the Pantanal, the largest wet plain in the work. From November to March, the rainiest season, a part of its land would be inundated by the waters that overflowed from the Taquari, one of the great tributaries of the Paraguay river, the largest body of water in the Pantanal. Between April and October, the dry season, rainfall diminishes and the portions of Aguapé that had taken a prolonged and reinvigorating bath would be dry once again. Year after year, it used to be like this, there and on so many other Pantanal farms, where the fauna, the flora and man were used to the cyclical coming and going of the waters. To the Pantanal’s so-called flood pulse. Eight years ago, the waters came in the flood season and no longer left 3,200 hectares of the farm. “This area is now permanently flooded”, says Emilio Cesar Miranda de Barros, Aguapé’s owner and the president of the Corumbá Rural Union, in Mato Grosso do Sul. To blame for the disaster: the River Taquari, which no longer keeps to its main bed, due to the accumulation of sediments, and keeps submerged, now during the 12 months of the year, an area of 6 thousand square kilometers of it basin, equivalent to four times the size of the city of São Paulo.

It is not just a question of a case of a silted-up river that escapes from its course and causes temporary and localized damages. It is more than that:  because of the material accumulated on the bottom of its channel, the Taquari, whose total length is about 800 kilometers, practically ceased to be navigable in its plain portion two or three decades ago, has less and less fish, and its waters burst at countless points the contention banks of its bed, invading important segments of land intended for cattle raising or that serve as dwellings for the fauna. In the last 30 years, there has been a resumption in the intensity of the rains that fall on the plateau part of the Taquari basin, a climatic condition that makes it even more difficult to keep the waters of the river in its lower stretch, inside the Pantanal, circumscribed to its current course.

Change at the mouth
At the moment, the most noteworthy destruction of the edges of the river is taking place in the environs of the Santa Luzia and Coronal farms, a stretch of the channel of the Taquari with higher levels of silting up. In these sectors, one part of the waters of the river remains in its main bed, while another, with a volume not to be despised, slips through the holes opened in the banks of the Taquari, falls onto the lowlands of the region of Paiaguás, and originates a succession of extensive new channels. In practice, from Coronal farm, the river has been divided into two. With time, only one Taquari will remain. The other, probably the one that runs along its current bed, is going to dry up. In the recent past, breaches in the banks of the river near to the region called Zé da Costa have made the mouth of the Taquari, which flows into the River Paraguay, change place. In 1973, the mouth was close to the locality of Porto da Manga, in the southwest of the region known as Nhecolândia. Today it is 30 kilometers upriver and, according to certain forecasts, may be displaced another hundred kilometers to the north in the next few decades, on the way to Lake Mandioré.

The progressive obstruction of the channel of the Taquari, which cuts from east to west the heart of a region that is unique in the world, is regarded by some scholars as the Pantanal’s biggest environmental problem, with negative repercussions as well for the local agribusiness and tourism. A problem, federal and complex. Federal, because the whole area of the Taquari basin, of roughly 79 thousand square, is spread over two states, Mato Grosso do Sul, covering 95% of the total, and Mato Grosso, the remaining 5% – and any intervention in the fate of the river needs the endorsement of Brasilia. Complex, because there are natural and human factors contributing for the river to have a restless disposition and its waters to always be looking for a new bed. “Due to the physical characteristics of its basin, the Taquari is a nomad river by nature, whose bed historically changes place from time to time, some hundreds or thousands of years”, says geologist Mario Luis Assine, from the São Paulo State University (Unesp), Rio Claro campus, who is studying this question in a project sponsored by FAPESP. “But the action of man in the plateau area of the river, located away from the sedimentary plain that characterizes the Pantanal, is speeding up this process.” For action, read deforestation and the activities of agriculture and above all cattle raising that started in these highlands back in the 1970’s.”?Cattle raising does not have a culture of handling the pastures, and for this reason favors the desegregation and erosion of the soil”, explains researcher Carlos Padovani, from Embrapa Pantanal, in Corumbá, the coauthor of studies with Assine and the coordinator of an ample diagnosis of the situation of the Taquari. “This material ends up going to the river.” In the region of Coxim, also on the plateau, about 36 thousand tons of sediments are discharged every day into the watercourse.

Wanderer by nature
A brushstroke of history and geography helps one to understand why the Taquari, a river essentially without a fixed bed, has become even more of a wanderer in the last three decades. Roughly one third of the watercourse, precisely its initial portion, from the source in Mato Grosso to a little after the locality of Coxim, in Mato Grosso do Sul, crosses plateau lands. In this stretch, around which the basin of the Upper Taquari is formed, the altitude of the terrain varies from a little less than 900 meters to about 200 meters. The river is well contained in its bed, but the deforestation and the erosion of the lands that make up its basin, an undesirable byproduct of the new farming frontier, produce tons of sediments that, taken by the current of the Taquari, are going to speed up the process of the watercourse silting up. After overcoming the scarp that marks the end of the plateau and the beginning of the sedimentary plain, the river enters into its marshy portion, in its middle and low courses that account for two thirds of its length. This less elevated portion of the basin – which forms the so-called giant Taquari alluvial fan, a geological feature molded by the transport of sediments of the river and easily recognizable in satellite images (it does indeed display the format of a large fan) – covers 50 thousand square kilometers, 37 % of the territory of the Pantanal in Brazilian lands. Here, the Taquari is a fragile system, laden with much sediment coming from the plateau, which runs in a main channel flanked by banks only slightly higher than its waters.

To complicate the situation even more, the lands around the current bed of the river are lower than its channel. It is as if, like an aqueduct, the Taquari were a river running through a suspended canal. This means that, after breaching the banks of the river, waters of the Taquari take hold of the neighboring lands, at less elevated altitudes, thus starting an endless quest for a new and more convenient bed. This process has been happening for thousands of years – and the proof of this is the existence of a network of ancient channels of the Taquari imprinted on the landscape of the Pantanal, beds on which the river flowed one day and today no longer flows. “The Taquari is a really unstable river”, explains  Padovani. Any change, of a natural or human order, or a combination of both, breaks the delicate equilibrium of the river. This, by the way, is one of the main conclusions presented in a scientific article by Assine, recently published in the international magazine Geomorphology.

From seminars and conversations with the local population and others interested in the subject, Embrapa Pantanal, the Alterra Institute, of Holland, and the Corumbá Rural Union formulated twelve different proposals for trying to minimize the permanent inundations arising from the silting up of the Taquari. Some suggestions are not exclusory and, if approved, may come to be adopted concomitantly. There are those who advocate, for example, the idea that the river ought to be dredged, a task that could cost between R$ 70 million and R$ 180 million and would take from two to ten years to be concluded. Others think that a dam ought to be constructed in the plateau area of the Taquari, to prevent the sediments from the highlands reaching the swampy lowland portion of the river. Reforesting the banks of the Taquari in its highland portion is an alternative that is remembered, as well as adopting a better handling of the soil on the plateau, in order to prevent erosion. For some environmentalists, an interesting measure would be to expropriate the areas that are today permanently inundated by the floods of the Taquari and to transform them into a national park.

There are even diametrically opposite propositions: one line of thought advocates the reconstruction of the burst spots of the banks of the Taquari, in particular near to the Coronal farm, for the river to stop flowing into its alternative channels and to go back to its central bed; another prescribes that man should adopt measures to the effect of making easier, and not more difficult, the natural tendency of the river to change course from time to time, making the Taquari adjust itself to its new bed in lower lands. There is no lack of arguments against and in favor of each one of the ideas. “For me, the only way to solve the problem is to reduce the erosion in the plateau area by means of proper handling of the soil, or perhaps speeding up the process of the river settling down in its new bed”, says Rob Jongman, from the Alterra Institute. “It would be important to adopt these measures, but they are not going to do away with the inundations of the Taquari”, ponders the geologist from Unesp. “Rivers in alluvial fans constantly change course.”

The Project
Current sedimentary dynamics and quaternary evolution of the alluvial fan of the River Taquari, Pantanal of Mato Grosso (nº 99/00326-4); Modality Regular Line of Research Grants; Coordinator Mario Luis Assine – Unesp, Rio Claro; Investiment R$39,205.00 and US$ 4,450.00

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