Preliminary draft.
LARGE RIVERS OF SOUTH AMERICA: toward the new approach
J.J. Neiff
Introduction
It is likely that water consumption for life maintenance is the most important of the multiple uses of water. In great part of the biosphere, water is very scanty, or it is even unattainable, and, what is even more serious, a progressively less amount of water can be devoted to direct consumption because of the increasing of pollution. Therefore, an increasing number of people counts on a less amount of water.
It is just as worrying that the loss of the water quality also affects all these forms of life in the biosphere, being it now expected a decrease in the biodiversity at the continents level, with impact on human populations, an even more difficult fact to measure.
Superficial waters are the most reachable for men, but only continental fresh waters allow its direct use for human, plant and pets consumption.
Diagrammatically, availability of superficial water at continental level depends on the positive water balance of rains and the landscape physiography to retain it, accumulate it, or allow its runoff towards the sea.
For an equal amount of superficial available water, continents have different proportion of cumulated water (bodies of lentic water) and running water (lotic environment).
South America is in a privilege situation since it has a greater volume of flow of superficial running waters, that result from the constant flow starting with some rain drops and reaching the sea. This fact ensures countries possibilities to dispose of "new" and clean water in the future, under the condition that the adequate use of the basins ecosystems can prevent the erosion processes, pollution caused by the pour of toxic substances, or by the effect of acid rains.
The design and adequate monitoring of the different programs of flood control in urban conglomerates, hydric exploitation for the energetic production, navigation and other possible alterations of the pulsatile regime is just as important.
South American's geographic singularity: large rivers
As Morello (1984) describes in his "South American ecological profile", the surfaces of the South American continent double those of Europe and reach the southern latitudes of the biosphere. However, owing to its triangular shape with it vertice looking south, its climate has a great oceanic influence, and as a consequence, it is warmer than the one of the same latitude in the northern hemisphere.
Orographically, South America is an asymmetric continent, with a continuous mountain chain (the Andes Mountain Range) the height of which reaches the troposphere and two shields or high cores: the Guayana and the Brazilian one. The remaining surface corresponds to big pieces of flatland with little concave surfaces, except in the austral Andes region and in a part of the Patagonia, where there is a great number of lakes of glacial origin formed in the pleistocene that are, currently, under a tempered climate.
Most runoffs in South America have sense and general W-E direction (Orinoco, Amazonas rivers) and most of the water and sediments moving along the continent are originated in the Andes mountains (of alkaline tendency, with a great amount of sediments of fine silt-sand and loam texture and a less amount of clay).
A minor amount of water runs-off with a predominant North-South direction (Paraguay, Paraná and Uruguay rivers) with neutral to slightly acid waters and unselected sediments (clay to coarse sand), coming from the geological erosion of the shield of Brazil.
According to the orographic origin of waters, and to biotic transformations that take place in vast flatlands, waters running along the rivers may be: "white water" (with a great amount of fine slit-sand and loam of the Andes Mountains); "black waters" (with few suspended sediments and a high content of dissolved and particulate organic matter); and "clear" (with intermediate characteristics). This simple classification developed by Sioli (1975) for the basin of the Amazonas river more than 30 years ago is currently applicable today to most large rivers in the continent.
Based on Troll's criteria to the north of Buenos Aires, the warm and wet maritime climates prevail (OAS, 1973). They receive their humidity and rains from the atmospheric circulation in the Atlantic ocean.
Resulting from this physiographic and climatic characteristics, South America is the continent where large rivers collect the greatest amount of superficial water to spill it into the Atlantic ocean. The three largest basins of the continent (Orinoco, Amazonas and Paraná), contribute with 13% of the total suspended solids delivered by all rivers to the oceans (Tundisi, 1994).
In the table 1 made with data provided by Welcomme (1985) and Neiff et al., (1994), is possible to see the importance of large rivers in South America compared to other continents.
Discharge (table 1, column 1), in relation to the surface of each basin, is always greater in South America than in other continents (see: rate 2/1 in the table).
When comparing the surface of continents, and the discharge of large rivers, it results that the amount of running water in relation to the continental surface is much greater in South America (Fig. 1).
Brief comparison of lakes and large rivers functioning in South America
In most South American lakes, superficial waters are of Pleistocene origin and have accumulated disturbances from geologic times, and the impact of antropic activities (control of water in tributary rivers, contaminants, sediments). In large South American river waters, only stays for some months flowing along the continent, thus carrying and transferring minerals and organisms through the basin.
Let's us exemplify, in lakes the percentage of the volume of water annually renewed is very low compared to the volume comprised in the basins. Water circulation is produced in one or more seasonal periods, and the mixture efficacy essentially depends on the physical attributes, specially in temperature. The periodic circulation of water depends greatly on the amount of solar energy that the mass of water receives locally.
Lakes, therefore, can be considered as systems with great potential energy and low kinetic energy. From the energetic point of view, they can be considered as "cummulators" with a slow active volume: the hypolimnion.
In South America, large rivers can comprise small and large lakes in their basin, as well as wetlands, other lentic environments, but the greatest volume of water is temporarily or permanently in horizontal movement. The water renewal rate, in a determined section, is high compared to cumulated water. The concentration of elements (nutrients, organisms, sediments) must be expressed in relation to the discharge values and not in volume units.
The greatest amount of power that goes through the system is kinetic and this is of great importance when analyzing the nutrients flow, temporal distribution patterns of organisms, the use and handling of rivers.
Physiographic differences and, particularly the land slope, can determine the presence of tributaries and quick runoffs and slow runoffs sectors. These latter do not have the typical characteristic of vectorial flow (typical of rivers) since they can run-off in one or other direction during the different periods of the year, depending on the volume of flow of the collector stream.
The table 2 shows some characteristics that enable to compare rivers with mountain vectorial landscapes to rivers with flatland landscape (equipotential) from the concept submitted by Gonzalez Bernaldez (1981).
Consequences of the movement of water in large rivers
The main difference as regards lakes is the horizontal movement of water and, besides, between small and large rivers is that in the former case, water moves only temporarily while in large rivers, flow is permanent and organizes the distribution patterns and the organisms abundance. They are, therefore, systems or macrosystems where water, nutrients, sediments and organisms pass through a certain runoff section at a certain speed.
For a better understanding of the functioning of the fluvial macrosystem variability, a "typical" lake (for example the Mascardi lake, Rio Negro, Argentina) can be compared to a dam lake (for example, Yaciretá dam on Paraná River at Argentina).
A very simple example made up with a glass (with the lake volume, relatively constant) and two tubes: one is the income of water and the other is the outlet of superficial water, can be applied.
Volume (v) comprised in the glass is the cumulated information (generally speaking) in a certain time (t). If water were not renewed (utopia) the internal organization would depend on the amount and quality of elements comprised in the glass (nutrient species, etc.), on the energy fluctuations that our "glass" (or water body) seasonally receives and on interactions of elements within the system.
In lakes:
Total internal change:
where:
P = Energy inflow (precipitation, solar energy)
E = Energy outflow (runoff, termal advenction, etc.)
S = Surface area
Q1 = Inflow of information (water, sediments, spp.)
Q2 = Outflow of information (water, sediments, spp.)
t = time
But in rivers:
Then:
Total turnover rate ®
Total turnover time TTt = 1/TTR
We would now place the income of water tube (nutrients, sediments, organisms) Q1 in the graphic, and the outlet tube that will bear the Q2 symbol. In this second example there is, besides the internal metabolism of the system, an inflow of information (nutrients, sediments, organisms) unit of time.
Normally, in lakes and rivers, the volume is relatively constant and the income and outlet volume of flow vary in an analogous way.
The turnover rate (TTR) is the percentage of the total water comprised in the glass that comes in or out in a certain period of time. The turnover time is reciprocal to the turnover rate and states the necessary time for a complete renewal of water in the glass.
If the glass has a 1 litre capacity and 100 ml income per day, the turnover rate will be 100/1000, or 0,1 or 10 per cent per day.
Both rates are of significant use in order to value the exchange of information of the system under analysis. In practice, the turnover time rate is generally used. The TTR would be different along the river sections. Values of nutrient concentration in rivers offer a small amount of information if not discussed with the volume of flow data crossing this point or section.
Turnover time for the Yacyretá dam (placed in the watercourse of the Alto Paraná) is about 3 weeks. This same assessment, made for the Mascardi lake, has an approximate value of five years.
Water renewal in large rivers (and of other elements of the system) is quite high as regards the information volume comprised in the system. For such reason, the indices application described in the system status cannot be the same to those used in the study of systems of low turnover (as it happens in most lakes).
The biocenosis analysis with the use of indices of dominance, abundance, equitability, diversity and other, of known use in low turnover time ecosystems (Hulbert, 1971), have a low use to indicate the organization complexity and the functioning of communities that live in large rivers. Most known indices express the organisms distribution in a number of species. The disadvantage that they have is that they do not incorporate the turnover-time and turnover-rate magnitude.
Going back to our example of the glass: an increase of 10 individuals (or species or information units, generally speaking) can give the same result even when the flow rate in the system varied in great manner. If the outlet rate (death, emigration) were 0, the change rate would be 10. But it would also be 10 if 200 individuals were incorporated, with a 190 exit, or if 1000 were incorporated and 990 got out.
These indices are not very sensitive to explain the changes in systems with a high populational turnover due to the horizontal movement of the water during floods in the river valleys. They can also end up in misleading conclusions since in several occasions the diversity is kept with small change even when there is a 60% renewal of the species forming the community between high waters and low waters phase (Frutos, 1993; Zalocar de Domitrovic, 1993). Still when comparing extreme situations of low waters and others with extraordinary flood, the specific diversity does not reflect significant contrasts. The use of the simplest similarity index (as the one of Sörensen) to different situations of extreme high and low waters state that similarity is less to 30% for phytoplankton (Fig. 2).
In biotic systems of rivers, mainly in those of high change rate as planktonic groups or those of invertebrates that live in plants the complexity analysis requires to know the change rate, the response time and the possibility for a population or community to repeat its structure throughout time (Poi de Neiff and Bruquetas, 1989; Huszar, 1994).
In order to graphically represent this idea: perception is completely different when we observe the blades of a fan which is not working (that is to say, without performing its essential function) and the one we obtain when the blades move at different revolutions per minute.
In our example of the fan, we should now think that each blade is a species (or population or bioform) and that our "fan" could have "X" number of blades (as many as different elements that form our community) each of them represented in a different color. Therefore, our perception "would have a different color" by the number of colors (species, elements) forming the blade, and the speed we give to the fan.
Yet, the problem gets more complex since in the case of rivers, changes are not produced in the form of cycles (bio-geo-chemical cycles are not cyclic within the system) and because flows are produced as energy pulses and matter pulses presenting flood phases and dry phases.
Both phases form the "hydro-sedimentologic pulse" (or simply "pulse"). And the way of these pulses is variable throughout the century, and even in a decade. Variability throughout the long series of time shows regularities that may be profitably studied by the use of the rescaled range analysis since it allows to find hydrologic variability tendencies (Armengol et al., 1991).
In rivers, the amount of water that goes through a certain section and in a certain unit of time varies generally in a sinusoidal way, as a consequence of the rain distribution, physiography and basin soils.