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Impoverished rivers

Hydroelectric power plants change the function of the Parana river and increase erosion along its banks

MARGI MOSSFrom the state of Minas Gerais to the River Plate: reservoirs reduce the flow of the Parana river MARGI MOSS

While they are being built, hydroelectric power plants dam the waters of rivers, flood towns and woodlands and force those who live close to the riverbanks to move. In exchange, they generate indispensible electric energy. Once they are ready, they also give rise to environmental problems, which are less well known but have just as strong an impact. Geologists and biologists from Parana and São Paulo examined the changes along the Parana river over the last 20 years and found that the hydroelectric power plant dams, on cutting across the river, also cut the water speed by 36% and the volume of suspended sediment by 70%.  The difference between the maximum and minimum water levels during the wet and dry seasons also became less, thereby changing how fish and other beings live.

Dams also give rise to daily tides. The floodgates close partially at night, when the consumption of power is lower, and reduce the level of water below the dam by almost one meter. In the daytime, the turbines need to produce more power, the floodgates allow more water through and therefore have the opposite impact. Studies coordinated by geologist José Cândido Stevaux, a professor at the State University of Maringá (UEM), in the state of Paraná, and of the University of Guarulhos (UnG), in the state of São Paulo, indicated that the daily fluctuation caused by the dams can increase erosion on the Parana riverbanks by 200%.

And there is certainly no shortage of hydroelectric power plants on this river: about 150, if one considers only those that have dams at least 15 meters high, on the river Parana itself and its tributaries, including the rivers Tiete, Grande and Paranapanema, which branch out over a 2.5 million square km area in Brazil, Paraguay and Argentina, forming the second largest web of rivers in Brazil. If, on one hand, these hydroelectric power plants produce 60% of Brazil’s power and supply regions that account for most of the population and economic activity in Latin America, on the other, they transform the Parana and its tributaries into a string of lakes that modify the behavior of the rivers.

“One year after the last hydroelectric power plant went on-stream, the stretch of the river closest to the dams turned into a swimming pool, it became so transparent,” tells us Stevaux, the coordinator of a group that has brought together experts from EUM, UnG, Paulista State University (Unesp) in Rio Claro, (in inner-state São Paulo) and from Argentinean universities and research institutes that are studying the Parana river. “Tourists love it, because they can dive and see rays and other fishing swimming really close.” At first, predatory fish, such as the one meter long dourado game fish probably liked it too. In clear water, they can see everything better and eat to their heart’s content. The problem is that those predators will get less and less to eat in the subsequent years, because the population of smaller fish will plummet.

The geologists and biologists in this group are focusing their research on one of the few Parana segments with no dams, between the estuary of the Paranapanema river, which separates the states of Sao Paulo and Parana, and the start of the Itaipu reservoir, which begins in the town of Guaira, Paraná, and extends over more than 120 km until it reaches the dams, one of them as tall as a 65-storey building. To measure the amount of sediment in suspension on this stretch of river, some 200 km long and 4 km wide on average, the researchers released a disk painted black and white and suspended by a rope in the middle of the river. The sooner the disk cannot be seen, the richer in sediment the river is. “A few years ago the disk would vanish from sight after 1.5 meters. Now it hits the riverbed, 4 meters under the surface, and we can still see it.” In this stretch of river, the transparency of the water is greater near the Porto Primavera power stations on the Paranapanema river. This is the largest artificial reservoir in the world, with a flooded area equal to seven times the area of the Guanabara bay in Rio de Janeiro.

The more transparent the water, the more sunlight it allows through, which changes the communities of plants and animals in the riverbeds. Microorganisms, fish and plants used to mire and darkness disappear. The algae, which depend on light, may grow not only on the surface, i.e., in their habitual area, but also at the bottom of the river. The danger is that they may multiply excessively, as was the case of the bivalve mollusk Limnoperna fortunei, an invading species that appeared in the last decade in the port of Buenos Aires, brought in the ballast water of ships coming from Asia. “With no predators, this mollusk spreads and does a lot of damage,” says Stevaux. It has even got in the way of the turbines of the Itaipu power station.

Through these studies, which include reconstruction of the river’s geological history, Stevaux’s team is expanding our understanding of tropical rivers, which have been studied less than those in temperate climates, whose flow depends on the melting of snow from mountains. Stevaux thinks that these studies will help to determine the acceptable limits of the environmental impact of the hydroelectric power stations that are yet to be built in the country. They have already helped the creation of the Ilha Comprida National Park and the Varzeas do Rio Ivenhema State Park, a tributary of the Parana river.

One of the group’s objectives is to determine the minimum water fluctuation in the river between the  dry season and when it is in flood, so as to reconcile the survival of fish and plants with the need to generate power. “As the dams result in water being stored, the rivers no longer flood and the water no longer gets to the lagoons where the fish lay their eggs. The grasslands along the river banks, which are flooded most of the time, only grow once the water goes down,” he exemplifies. “These changes in the water flow can spread and radically change the entire environment.”

According to Stevaux, scientific articles and masters degrees and doctoral papers generated thanks to this research are contributing to defining and managing  tourist activities near the great rivers of the Parana basin, by indicating how much exploitation a given area can take. His team prepared a mathematical equation that established 12 levels of environmental fragility and he concluded that the secondary rivers in the towns of Porto Rio, in the state of Paraná, Taquarussu, in the state of Mato Grosso do Sul, and Rosana, in the state of São Paulo, are close to their maximum environmental impact (level 10), as a result of being heavily visited by fishermen at the end of the year.

However, the  dams and the artifical lakes do not only cause problems. They also foster river tourism, which draws those who live around the cities of Presidente Prudente and Maringa, and they give rise to challenges, such as defining the areas for tourism. The beaches formed by the river move: one year they may be 200 meters away from the end of a town, and the following year, three kilometers away. Another challenge is sand mining – it is unclear exactly how much one can take without harming the river. “We plan to help prepare protection laws covering river estuaries, and not only their sources, which are already protected,” said Stevaux.

The project
Propagation of the “impact wave” on the dynamics of flow and riverbed load along the Parana river. Model for managing alluvial rivers under the impact of dams, waterways and mining (nº 04/14057-5); Type Regular Research Awards; Coordinator José Cândido Stevaux – University of Guarulhos; Investment R$ 130,000.00 (FAPESP) and R$ 220,000.00 (CNPq-ProSul).

Scientific article
STEVAUX, J.C. et al. Changes in a large regulated tropical river: the Paraná River downstream from the Porto Primavera Dam, Brazil. Geomorphology. v. 110 (in press).