How to argue about the Guaíba
Outline
In the previous post I evaluated how not to argue about the Guaíba, in the context of the discussion on whether this water body is a river or a lake. Following up, I will now move forward with the basic principles of a rational discussion on the subject, and why I understand the Guaíba to be a lake.
Read also: How not to argue about the Guaíba
Deductions, Gaúchos and controversies
The argumentative fallacies listed in the previous post are wrong because they violate the logic of deductive inference.
A deduction has the following structure: (1) a conditional premise is pre-established; (2) an antecedent statement arises, and; (3) a consequent inference is born. For example:
- premise: it is true that all gaúchos like a controversy;
- antecedent: it is true that Joana is a gaúcha;
- consequent: then it is true that Joana likes a controversy.
The consequent sentence is implied by the premise and the antecedent. In the case of fallacies, it is not implied, for example, that the Guaíba is a river or a lake just because the population prefers it one way, or because the law protects one or the other more.
False hypotheses
The philosopher Karl Popper teaches us that deduction is very useful in Science, as the premise consists of a hypothesis and the antecedent consists of refuting empirical evidence. For example:
- hypothesis: all gaúchos like controversies;
- refuting evidence: Mateus is a gaúcho, but he doesn’t like controversies;
- conclusion: then the hypothesis that all gaúchos like controversies is false.
The confirmation of a hypothesis is much more complicated, as specific confirmatory empirical evidence does not guarantee the truth of a broader hypothesis:
- broad hypothesis: all gaúchos like controversies;
- specific confirmatory evidence: José is a gaúcho and likes controversies;
- conclusion: then the hypothesis remains open, as it refers to all gaúchos, not just José.
To confirm the hypothesis, all gaúchos would need to be interviewed, whereas just one gaúcho who doesn’t like controversies is enough to demonstrate that the hypothesis is false.
One thing is one thing, another thing is another thing
The question of whether the Guaíba is a river or a lake is obviously a geophysical problem. But, above all, it is a classification problem.
This type of problem demands that a collection of substantive objects be instantiated a priori, defined in an exhaustive and mutually exclusive way. That is: cats cannot be dogs at the same time.
In the same way, rivers cannot be lakes. Lakes cannot be estuaries. One thing is one thing, another thing is another thing.
Classification is a way of symbolically encapsulating the complexity of reality. Despite simplifying things, this is, in fact, the great objective of language: giving up details to gain communicative capacity.
If I had to explain in minute detail the bizarre sensations that plague my stomach every time, I would never have lunch. It is much more practical to say “I am hungry”.
The universe of equations
Some might claim that the “geo” part of “geophysics” is irrelevant, and only the “physics” part matters. Engineering colleagues prefer this approach, while natural scientists insist on working with qualitative concepts and classes.
From the perspective of physics, it is only necessary to instantiate a universe with a few very simple things, such as the concepts of time, space and mass; Newton’s three laws; and perhaps some constants, such as the acceleration of gravity and the viscosity of the fluid. The result of this is the Navier-Stokes equations:
$$ \rho \left( \frac{\partial \mathbf{u}}{\partial t} + \mathbf{u} \cdot \nabla \mathbf{u} \right) = - \nabla p + \mu \nabla^2 \mathbf{u} + \mathbf{f} $$
where $\rho$ is the fluid density (kg/m³), $\mathbf{u}$ is the velocity vector (m/s), $t$ is time (s), $p$ is pressure (Pa), $\mu$ is dynamic viscosity (Pa·s), $\nabla$ is the gradient operator, and $\mathbf{f}$ represents external forces (N/m³). These equations can be simplified in different situations, such as for shallow water flow.
The formulas of fluid mechanics do not make the slightest distinction between river, lake, estuary, etc. But, by solving them, it is possible to determine the velocity of the water in time and space, which is much more useful information for theoretical and practical issues.
An algorithm
But if we are forced to classify the Guaíba to facilitate our communication, then it is necessary to follow a very clear script and apply deductive logic to a set of classes based on empirical evidence. The steps can be formulated as an algorithm:
- choose a system of classes;
- collect empirical evidence about the object;
- for each class, test the evidence against the class definition;
- if the evidence contradicts the class definition, reject that class.
Why the Guaíba is not a river
With this protocol, I see that the Guaíba cannot be a river because the class of “river” fails the test in any reasonable classification system. I will explain.
Meanders, pools and rapids
For example, if we adopt the classification of rivers proposed by David Rosgen (1994), one of the parameters necessary to classify rivers is the slope of its channel.
In this line, we can define a river as follows:
“A river is a drainable channel.”
As such, rivers create fluvial systems of erosion, transport and deposition. In steeper sections, they form pools and rapids. In flatter areas, bars (deposits) and banks (erosion) are created. Meanders avulse on sedimentary plains, resulting in natural levees and oxbow lakes (abandoned meanders).
Furthermore, rivers are fed by rains. Rivers are perennial to the extent that the soils on the slopes can store rainwater and release it slowly during dry weather. When it doesn’t rain for many months, the level of the rivers drops until the water simply disappears. In other words, rivers can dry up.
However, in the case of the Guaíba, the bathymetry evidence released by Nicolodi and colleagues (2011) points out that there is no effective slope, as its bottom is below sea level. In some places, the bottom is as much as 10 metres deep below the level of the Ocean.
Therefore, if it never rained again, the Guaíba would not dry up. In this hypothetical situation, the average level would drop by about one metre from the current average, causing a few shallows to emerge on its banks.
During an eventual extreme drought, therefore, we run the risk of seawater intrusion into the Guaíba, which would affect the water intake of millions of people. This process of saline intrusion occurs frequently in the Laguna dos Patos, but normally does not reach the Guaíba. The eventual opening of more channels in the Estuary, or the rise in sea level, could change this dynamic (for the worse).
Everything is a river
Many colleagues defend the river classification by evoking that there is a substantial flow current that crosses the Guaíba from one end to the other:
“There is a flow current in the Guaíba channel. Therefore, the Guaíba is a river”
The appeal to flow is a legitimate argument, as it is geophysical. However, in my opinion, this argument incurs a category mistake, as it confuses the definition of flow current with the definition of river. That is, it establishes that:
“A flow current is a river”
Of course, a river is a flow current. But not every flow current is a river.
By reversing the order, somewhat weird conceptual consequences are created. For example, to be consistent, we would be forced to establish that the Laguna dos Patos Estuary is a river, as there is a large current of water flow in this region.
In the Ocean there are several flow currents, which would then be rivers. Even turbidity currents would be rivers. Similarly, flow currents entering artificial dam lakes would, by definition, also be rivers.
The Guaíba Lake
The Guaíba also could not be an estuary or a lagoon, as these definitions involve the constant presence of Ocean salinity, a fact that does not occur in current conditions.
We are somewhat forced, thus, to adopt the class of lake
This class is what remains for us. And this is how the deductive process in Science works: hypotheses survive the tests of empirical evidence. Below are some of these evidences, released by researchers from the Centre for Coastal and Oceanic Studies (CECO/IGEO).
The bottom of the Guaíba is dominated by silt
Studies conducted with DMAE in the late 90s brought ample evidence about the bottom sediments of the Guaíba. The conclusion is that the sand content in its centre, precisely where the famous flow current passes, is from 0 to 10%.
In this area, most of the sediment is silt, a muddy and fine fraction of sediment, which needs calm waters to deposit. This implies that, even with the flow current crossing the lake, calm conditions predominate for deposition below the surface.
The sand content increases around the mouth of the Jacuí Delta and on both margins of the lake, where it can reach 100%.
Erosion on the banks occurs through wave action, generated by winds
A hydrodynamic modelling study, conducted by Nicolodi and colleagues (2013), assessed that the sand content on the margins is explained by the action of waves, which are generated by winds. Waves, thus, cause the resuspension of fine silt particles, leaving only the coarser fraction of sand behind.
This modelling study generated a map of sedimentary environments of Guaíba Lake. The authors suggest that in the centre of the Lake, even with the flow current, it is a predominantly depositional region.
There is a large flow current… but winds can reverse its direction
Negative flow
An interesting observation, published in 2020 by Scottá and colleagues, is the reversal of the flow current of water at the mouth of the Jacuí Delta (channel in front of the Gasômetro Plant).
They measured the flow with sensors along the channel section, showing that generally the flow there is intense and positive, bringing new water from the Delta into the Lake. But when the south wind is strong enough, the current reverses, and there is a negative flow of water, which flows from the Lake into the Delta.
This phenomenon is long known by the operators of the Ilha da Pintada WTP, who know very well, in practice, this flow variability associated with the south wind. In this case, it is not necessary to measure the flow with sensors in the water, as the water quality itself changes drastically when a reversal occurs, which forces the operators to change their purification processes for treatment.
These evidences corroborate that the Guaíba is a lake and help to refute that the Guaíba is a river. Any other eventual definition for the Guaíba would need to be simple enough and also explain all these observations. The lake classification does the job.
References
David L. Rosgen, A classification of natural rivers, CATENA, Volume 22, Issue 3, 1994, Pages 169-199, ISSN 0341-8162, https://doi.org/10.1016/0341-8162(94)90001-9.
Bachi, Flávio, Barboza-Pinzon, Eduardo, and Toldo, Elirio (2000). Estudo da Sedimentação do Guaíba. Ecos, 17, 32-35. https://doi.org/https://www.researchgate.net/publication/305827830_Estudo_da_Sedimentacao_do_Guaiba
Nicolodi, Joao L., Toldo, Elirio, and Farina, Leandro (2011). Wave dynamics and resuspension in Lake Guaíba (Brazil) with implications on points of water abstraction for human supply. Journal of Coastal Research, SI 64, 1550-1554. https://doi.org/https://www.jstor.org/stable/26482435
Nicolodi, Joao Luiz, Toldo, Elirio E., and Farina, Leandro (2013). Dynamic and resuspension by waves and sedimentation pattern definition in low energy environments: guaíba lake (Brazil). Brazilian Journal of Oceanography, 61, 55–64. https://doi.org/http://dx.doi.org/10.1590/S1679-87592013000100006
John Grotzinger, Thomas H. Jordan (2014). Understanding Earth 7th Edition. Macmillan Learning UK.
Scotta, Fernando Comerlato, Andrade, Mauro Michelena, Weschenfelder, Jair, Toldo, Elirio Ernestino, and Nunes, Jose Carlos Rodrigues (2020). Descarga líquida e sólida em suspensão no Rio Guaíba, RS, Brasil. Pesquisas em Geociências, 47, 19p. https://doi.org/10.22456/1807-9806.109983
Other resources
Statement at the public hearing on May 30th:
Possantti, I. (2025, May 31). I think that the Guaíba is not a river. Lake is a good classification. And I will only change my mind with a good geophysical argument. (statement at public hearing). Zenodo. https://doi.org/10.5281/zenodo.15565330