The water flow in the region downstream of the Tucuruí dam showed an increase in its variability over the years, with this increase being accompanied by a greater number of extreme events of flooding and flow of the river (over short periods of time, caused by the operation of the dam). This situation worsened from 1997 to 2000, a period in which there was an acceleration in the tendency to increase. This result shows that the river's natural flood cycle is gradually being superseded by a periodicity related to power plant energy production. This periodicity, in turn, would have a much shorter frequency, where peak flows can have intervals of up to one day. Such a replacement of the cycle is something that has already been documented for other hydropower plants in various regions of the world, which This result shows that the river's natural flood cycle is gradually being replaced by a periodicity related to power plant energy production. This periodicity, in turn, would have a much shorter frequency, where peak flows can have intervals of up to one day. Such a replacement of the cycle is something that has already been documented for other hydropower plants in various regions of the world, which in turn has direct responses on both the biological communities and the human populations living along the river banks (Poff and Olden 2017).
A relevant and differential point in our study relates to the governmental dynamics that Brazil has been employing for energy production. The interconnection of the Tucuruí power plant to the SIN in 1997 appears to be a landmark in the trends observed in our results. As of this year, the divergence of the flow dynamics from what would be expected in a natural environment accelerates, maintaining a very high rate of change. In addition, as energy demands become greater and greater, as the power plant begins to serve a larger distribution network, energy production undergoes moments of greater intensity, which leads to the release of a greater volume of water. At the end of this process, increased demand-increased production-greater release of water volume, the externality is for the downstream part of the river that suffers large flooding events at a very fast rate, accompanied by rapid receding.
The use of natural resources has always been accompanied by externalities events, either directly or indirectly (Fearnside 2014). There is a vast literature that demonstrates the local effects that human modifications generate to natural systems, and also for traditional populations (Walters and Post 2011, Ruhí et al. 2012, Burton et al. 2020). High impact actions are normally expected to have effects, usually negative, in a short period of time and limited spatial range (Burton et al. 2020). However, several current contexts concerning the means of production, distribution and consumption by human societies have widened the scope of impacts. We can highlight that the current major concern of the scientific and governmental community is related to the effects of human action on global climate change (IPCC 2014). The scenario reported in our study falls within this context, in which the actions of human populations create adverse conditions in very remote regions.
The new reality of the downstream stretch of the Tocantins River after the Tucuruí dam is a deleterious environment for the biological communities living in the river canal, particularly fish and crustaceans, as well as for the riverside populations living along its banks that depend on fishing resources. When the river level rises too rapidly, many organisms tend to move to the margin regions (floodplains). When it is no longer necessary for the turbines to raise the water level for energy production, the affluent flow in the dam is again reduced, which leads to an accelerated ebb event in the downstream part of the river. At this point the river reduces its water column to the levels considered normal for the operation of the power plant, and the entire floodplain region, previously flooded, quickly dries up. Consequently, with this accelerated withdrawal of water from the floodplain, many organisms that have moved into this environment end up being trapped in temporary pools, which usually dry out quickly, thus generating a high mortality of fish and crustaceans.
Many riverine populations have pointed out this problem of animal mortality due to sudden changes in the flow downstream of the dam. Despite the fact that riverbank dwellers have reported the high mortality rate and documented it in videos (see video DOI: 10.5281/zenodo.6539935), no study has yet been established to estimate the biomass and number of dead animals. Even without these numerical estimates, we see that this value can be very high according to the documents presented by the local people. Most of the animals that die in this process of rapid filling followed by a rapid ebb are species of economic and sustainability value for these riverbank populations. The loss of these organisms can then be seen as both an environmental impact (loss of species) and a socioeconomic impact (loss of vital resources to traditional populations).
The future outlook for the region downstream of the Tucuruí dam tends towards a scenario where the alterations in the dynamics of the flooding and ebb tide of the river will aggravate. In recent years, Brazil has faced successive water crises, where the shortage of rainfall in the South and Southeast regions has reduced the reservoir levels of the main hydroelectric plants in these regions (Hunt et al. 2022). We can report that the main factors related to this increase in rainfall deficit are the increase in deforestation in Brazil, especially in the Amazon region, and global climate change (de Almeida et al. 2010, Almeida et al. 2016). With the low power generation capacity of these plants close to industrial centers and large cities, the need for more distant plants to supply this energy demand is increasing. Since the Tucuruí HPP has one of the largest water reservoirs in the world, this hydropower plant is a strategic resource, since its production capacity is capable of meeting greater demands. From this point of view, we can speculate that the tendency is for a steady increase in downstream flow variability, as well as the increase in extreme cases of variation in the height of the river's water column.
The results of our study indicate a need for greater participation of local populations for control and decisions about the management of water flow in the reservoirs. Measures such as advance warning to these populations about possible events of major floodgate opening, which would lead to a large filling of the downstream part of the river, may be desirable, but are not enough to minimize environmental damage. The warning would make it possible decision making that would minimize the deleterious effects that these rapid floods have on fish stocks, thus preventing the mortality of these animals from being wasted. In addition, for the governmental decision-making sphere, it would be preponderant that the energy demands coming from the national integrated system did not overload only one generating unit. It is necessary to have a governance of the energy system in which there is an integrated vision with the local and regional socioenvironmental issues where the hydroelectric plants are located, especially in the Amazon region. In such a region there must be a greater concern with its sustainability that is so important for the planet. To this purpose, the establishment of a system-wide strategy of diffuse demand will make it possible for these events of intense energy production by the hydroelectric plants to have a less steep production acceleration curve (and thus reduce the speed of floodgate opening), since a large number of plants would be feeding the demand.
Another mechanism that can cushion the effects of flow control in large rivers, especially during the dry season, is an increased use of solar energy, thus setting up a hybrid system of energy production and distribution (Mouriño et al. 2016). Such measures would enable a large reduction in the impacts that long-distance activities would have on traditional populations and environments where electricity is generated. We emphasize that with the advent of tools that increasingly connect distant regions, we may have to replace the maxim "think globally and act locally" with "think globally and act globally too."