Meeting of the Waters, near Manaus, Brazil
  


Andes, Hylean, and Craton:
The faces of the rivers


Samara Salamene,
and
Victor M. Ponce

13 February 2020


Abstract. The various tributaries of the Amazon river may be generally classified according to the color of their waters into three distinct types: (1) white, (2) black, and (3) clear. This study features the following major tributaries: Solimões, with whitewaters; Negro, with blackwaters; and Tapajós and Xingu, with clearwaters. The different color of the waters is explained in terms of the geological, geomorphological, hydrological, and ecological characteristics of their respective drainage basins.


1.  CLASSES OF AMAZONIAN RIVERS

The Amazon basin is the largest in the world, encompassing approximately 7.5 million km2 in South America. The basin is also the largest source of freshwater; with a mean discharge of approximately 220,000 m3/s at its mouth, it constitutes about 1/6 of all the fresh water that flows into the oceans (Ponce, 1992). Located in the tropics and mostly covered by forests, 68% of the basin area lies within Brazil; the remaining 32% covers parts of the following eight countries: Bolivia, Colombia, Ecuador, French Guiana, Guyana, Peru, Suriname, and Venezuela.

The waters of Amazon rivers feature different colors, which are readily discernible. Native pre-Columbian inhabitants classified the rivers attending to the color of their waters. They knew that the color of the water indicated differences in its quality, fish resources, soil fertility, and the presence/absence of mosquitos. Subsequently, European settlers used river color to classify Amazon rivers into four types: (1) Clear river (Rio Claro), (2) Black river (Rio Negro or Rio Preto), (3) White river (Rio Branco), and (4) Green river (Rio Verde) (Furch and Junk, 1997).

The first attempt to develop a scientific classification of Amazonian rivers was performed in the 1950s by Harald Sioli (Junk, 2001). He used the color of the waters, as well as various physical and chemical characteristics, to explain the limnological properties of major Amazonian rivers. Sioli related these characteristics to the geological and geomorphological properties of the contributing drainage basins (Fig. 1) (Junk et al., 2011).

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Fig. 1  Spatial distribution of major Amazon basin blackwater, clearwater,
and whitewater tributaries (Junk et al., 2011).

Sioli established three types of water color in major Amazonian rivers: (1) clear, (2) white, and (3) black (Fig. 2). More recent hydrochemical data indicates that the chemical composition of Amazonian water bodies is much more complex than that originally envisioned by Sioli. However, due to its simplicity, Sioli's classification remains valid (Rios-Villamizar et al., 2014).

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Fig. 2  Clearwater, whitewater and blackwater river colors.

The color of a river's water is the result of physical and chemical transformations which take place during surface and subsurface runoff. In the Amazon basin, whitewater draining from the Andes Mountains (for example, the Solimões river in Brazil, called Amazon river in Peru) has a relatively high concentration of nutrient-rich sediments; thus, its light-brown color. On the other hand, blackwater, originating in the central-northern Hylean rainforest (for example, the Negro river), has a high concentration of humic substances (partly decomposed organic matter), which gives it a characteristic dark (black) color. Furthermore, clearwater (for example, the Curuá Una river), draining the neighboring Brazilian shield region (craton, i.e., tertiary rocks), is patently poor in sediments and, therefore, transparent, its chemical composition resembling that of rainwater (Fig. 3).

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Fig. 3  Selected physico-chemical properties of Amazonian rivers: (a) specific conductance (at 20 °C) and pH;
and (b) distribution of important alkali (Na, K) and alkaline earth (Mg, Ca) metals (Furch and Junk, 1997).


2.  THE SOLIMÕES RIVER: WHITEWATER

Most western Amazonian rivers are classified as whitewater (Fig. 1). The white waters are generally muddy, containing large quantities of sediment and often having a brownish tint. The mainstem Amazon river, the Solimões, is classified as whitewater; important tributaries such as the Juruá, Purus and Madeira are also whitewater.

The headwaters of whitewater rivers lie toward the west, in the Andes Mountains of Ecuador, Peru, and Bolivia and carry large amounts of nutrient-rich sediments, giving the water its characteristic light-brown color. In addition, at the prevailing (high) temperatures, the mainly alkaline earth metals and carbonates determine the muddy coloration of whitewaters, with a relatively high value of electrical conductivity. In the upland basins, this value is approximately 100 μS cm-1, decreasing to 40 μS cm-1 in the lowland basins. Moreover, the pH of whitewaters is close to neutral (Furch and Junk, 1997).

These rivers deposit their nutrient-rich sediment in extensive floodplain areas, referred to as várzeas in the Portuguese language. Therefore, the várzeas are very fertile and covered by highly productive terrestrial and aquatic herbaceous communities, which reach beyond the várzea forests.

Sediments carried by whitewater rivers consist of large amounts of fine-grained material. This material increases water-holding capacity during a dry phase, but it also impedes soil aeration. Large roots dig deep into the mud, drying during a recession phase can damage the root systems. The clay fraction contains kaolinite, illite and smectite. Unlike kaolinite, smectite has a high ion-exchange capacity and it releases potassium over time. Both kaolinite and smectite are essential for the fertility of floodplain soils (Junk et al., 2011).

In mountainous environments, physical weathering is more likely to predominate over chemical weathering. Thus, physical weathering of the Andes Range conditions the geochemistry of downstream tributaries. Predictably, in the Amazon basin, about 84% of the total amount of dissolved and suspended solids originates in only 12% of its contributing area, located to the west (Rios-Villamizar et al., 2014).


3.  THE NEGRO RIVER: BLACKWATER

The blackwater rivers, among them, the Negro, Jutaí, Tefé and Coari rivers, feature much darker tones. This is due to the soil chemistry and to the local geology, geomorphology, and hydrology. Blackwaters are poor in nutrients and the surrounding soil is predominantly sandy, containing large amounts of organic matter, such as humic and fulvic acids, which lend the water its characteristic black color.

Although the water surface appears dark in color, collecting the river water in a transparent bottle will reveal a color tint varying from red to dark brown. The red waters found in the Negro river at the city of São Gabriel da Cachoeira, in the central-northern Amazon, fall into the black water category because they are acidic waters containing a high quantity of humic acids, which give it a reddish-brown color (Gibbs, 1967).

Most headwaters in the northwestern Negro tributaries have transparent waters with up to 3 m Secchi depth, featuring low amounts of suspended matter. They drain the waters originating in the Precambrian Shield of Guyana, characterized by large areas of white sands (podzols). This is the case of the Branco river, a tributary of the Negro river, which has a high load of suspended matter and the appearance of a whitewater river. However, the chemical characteristics of these rivers indicate that they generally have a low nutritional status and a closer relationship with clearwater rivers. They become blackish in color and very acidic after flowing through areas covered by dense rainforest (Junk et al., 2011). The German naturalist Alexander von Humboldt referred to these forests as Hylean in the 19th century.

Blackwater rivers feature pH values in the range 4-5 and low electrical conductivity, below 20 μS cm-1. They mainly transport the sandy bedload and a small fraction of low fertility kaolinite. The water is acidic and the amount of dissolved inorganic substances is small (Furch and Junk, 1997). Secchi water transparency is about 60 to 120 cm, with low amounts of suspended matter and high amounts of humic acids, which give the water a reddish-brown color. The amount of dissolved humic substances is about ten times greater than whitewater Amazon rivers. The water is poor in nutrients and electrolytes, with the predominance of sodium among the major salt cations (Gibbs, 1997).

The floodplains of blackwater rivers, locally referred to as igapós, feature generally low fertility, Terrestrial and aquatic herbaceous plants are scarce and many typical whitewater species are absent due to either low fertility, low pH, or both. Sandy beaches suffer severe drought stress due to their low water retention capacity. Downstream of the Negro River, near the confluence with whitewaters and a greater availability of nutrient-rich sediments, there is a larger number of plants (Fig. 4) (Junk et al., 2011).

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Fig. 4  Meeting of the waters of the Negro river (blackwater, right) and Solimões river (whitewater, left).


4.  THE TAPAJÓS AND XINGU RIVERS: CLEARWATER

The largest Amazonian rivers featuring clearwaters are the Trombetas, Tapajós and Xingu rivers. Clearwater rivers usually have tones varying from greenish to transparent. They originate in the Amazonian cratons, i.e., very old rock formations dating from the Archean period (Pre-Cambrian); therefore, they have very small amounts of sediment (Junk et al., 2011).

Cratons exist in two regions of the Amazon basin: (1) toward the north, as the Guiana (Guayana) shield, and (2) towards the center-south, as the Brazilian shield (Fig. 5). These rock formations underlie a characteristically flat relief, with little surface erosion and low quantities of organic matter; consequently, the waters are clearer (Gibbs, 1967).

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Fig. 5  Generalized map showing the relief of the Amazon Basin (Silva et al., 2013).

Large clearwater rivers have an electrical conductivity ranging from 10 to 50 μS cm-1, which may decrease to 5 μS cm-1 for lower-order streams. The pH is acidic, ranging from 5 to 7, while the transparency of its greenish waters is over 100 cm and may exceed 350 cm.

The floodplains of clearwater rivers, also referred to as igapós, are generally of intermediate fertility (Junk et al., 2011). They are covered by a slow-growing floodplain forest, where litter production is approximately 30% lower than in other forested areas. The growth rate of trees in the igapós are up to two-thirds lower than those found in the várzeas (Furch and Junk, 1997).

Clearwater rivers receive their water mostly from rainfall, with little or no sediment production in the contributing uplands. Their wetlands are generally poor in nutrients; however, their nutritional status may vary due to differences in soil quality in the neighboring cerrados, the tropical savannas of central Brazil. Submerged macrophytes may occur in areas featuring deep light penetration and little water level fluctuation. Thus, the diversity of aquatic macrophytes is greater in clearwater rivers than in comparable whitewater or blackwater rivers (Rios-Villamizar et al., 2014).


5.  THE FACES OF THE RIVERS

The Amazon basin as a whole features three major landscapes: (1) the Andes Mountains, toward the west; (2) the Crystalline Shields of Guyana, toward the north, and Brazil, toward the southeast; and (3) the Sedimentary Plain in the central portion, demonstrably the domain of the Hylean forest. The geological characteristics of the lands where these landscapes are located determine the chemical composition of the waters of the various rivers that make up the Amazon basin. Not only are the chemical characteristics quite distinct, but there is also a visual difference, which led Sioli to classify them into: (a) clearwater, (b) whitewater, and (c) blackwater (Table 1) (Silva et al., 2013). Sioli's classification has been supported by botanists and limnologists, who found corresponding differences in the occurrence of tree species, aquatic macrophytes and aquatic biota, such as snails, bivalves, and other species (Junk et al., 2011).

Table 1.  Average values of pH, color, turbidity, dissolved oxygen, and ammonium ion
in selected Amazonian rivers.
River pH Color
(mgPt/L)
Turbidity
(NTU)
D.O.
(mg/L)
NH4+
(mg/L)
Solimões (upstream) 7.5 53.86 165.36 3.03 0.03
Solimões (downstream) 6.86 77.42 50.05 4.48 0.31
Negro (upstream) 4.65 45.23 3.03 4.4 -
Negro (downstream) 4.95 129.59 5.14 4.36 0.31
Tapajós 6.71 18.48 2.26 7.03 0.1
Xingu 6.98 9.48 1.69 8.94 0.1

The combination of various dissolved solids and chemical parameters, such as the amount and relationship between alkaline and alkaline earth metals, and major anions (bicarbonates and chlorides), electrical conductivity, pH, total nitrogen, water color, turbidity and transparency, enable the identification of three classic types of water (white, black and clear) and other mixed waterbodies of intermediate color. As the order of a river increases, the complexity tends to be hidden, because the river flow provides the integration of all types of waters throughout the watershed, thereby mixing waters of different qualities. The distribution of alkaline metals, alkaline earth metals, and of major anions is particularly useful for distinguishing between whitewater, blackwater and clearwater rivers. Greater variability is shown by bodies of water that do not fit into these three classic categories. Therefore, many rivers and streams should be considered as "mixed waters," resulting from the influence of lower-order tributaries with different physicochemical properties (Gibbs, 1967).


REFERENCES

Furch, K. and W. J. Junk. 1997. Physicochemical conditions in the floodplains, Chapter 4 in W. J. Junk, ed., The Central Amazon Floodplain. Ecological Studies, 126, 69-108, Springer-Verlag, Berlin.

Junk, W. J. 2001. Appraisal of the scientific work of Harald Sioli. Amazoniana, 16(3), 285-297.

Junk, W. J., M. T. F. Piedade, J. Schongart, M. Cohn-Haft, J. M. Adeney, and F. Wittmann. 2011. A classification of major naturally-occurring Amazonian lowland wetlands. Wetlands, 31, 623-640.

Gibbs, R. J. 1967. The geochemistry of the Amazon river system. Part I: The factors that control the salinity and the composition and concentration of the suspended solids. Geological Society of America Bulletin, 78, 1203-1232.

Ponce, V. M. 1992. Letters to South American Explorer, 31, May 1992.

Ríos-Villamizar, E. A., M. T. F. Piedade, J. G. Da Costa, J. M. Adeney, and W. J. Junk. 2014. Chemistry of different Amazonian water types for river classification: A preliminary review. Water and Society II, 178, 17-28.

Silva, M. S. R., S. A. F. Miranda, R. N. Domingos, S. L. R. Silva, and G. P. Santana. 2013. Classification of Amazonian rivers: A strategy for the preservation of these resources. Holos Environment, 13(2), 163-174.


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