Blackwater river

A swamp-fed stream in northern Florida, showing tannin-stained undisturbed blackwater.

A blackwater river is a type of a river with a deep, slow-moving channel flowing through forested swamps or wetlands. As vegetation decays, tannins leach into the water, making a transparent, acidic water that is darkly stained, resembling tea or black coffee. Most major blackwater rivers are in the Amazon Basin and the Southern United States. The term is used in fluvial studies, geology, geography, ecology, and biology. Not all dark rivers are blackwater in that technical sense. Some rivers in temperate regions, which drain or flow through areas of dark black loam, are simply black due to the color of the soil; these rivers are black mud rivers. There are also black mud estuaries.

Blackwater rivers are lower in nutrients than whitewater rivers and have ionic concentrations higher than rainwater.^[1]^[2] The unique conditions lead to flora and fauna that differ both from whitewater and clearwater rivers.^[3] Where the water types combine is attractive to a diverse group of organisms.^{[citation needed]} The classification of Amazonian rivers into black, clear and whitewater was first proposed by Alfred Russel Wallace in 1853.^[3]

Comparison between white and black waters

Table 1: Mean ionic composition, specific conductivity (μS/cm), and pH in Amazon waters.^[4]
	Solimões or Amazon River – whitewater.	Rio Negro – blackwater.
Na (mg/L)	2.3 ± 0.8	0.380 ± 0.124
K (mg/L)	0.9 ± 0.2	0.327 ± 0.107
Mg (mg/L)	1.1 ± 0.2	0.114 ± 0.035
Ca (mg/L)	7.2 ± 1.6	0.212 ± 0.066
Cl (mg/L)	3.1 ± 2.1	1.7 ± 0.7
Si (mg/L)	4.0 ± 0.9	2.0 ± 0.5
Sr (μg/L)	37.8 ± 8.8	3.6 ± 1.0
Ba (μg/L)	22.7 ± 5.9	8.1 ± 2.1
Al (μg/L)	44 ± 37	112 ± 29
Fe (μg/L)	109 ± 76	178 ± 58
Mn (μg/L)	5.9 ± 5.1	9.0 ± 2.4
Cu (μg/L)	2.4 ± 0.6	1.8 ± 0.5
Zn (μg/L)	3.2 ± 1.5	4.1 ± 1.8
Conductivity	57 ± 8	9 ± 2
pH	6.9 ± 0.4	5.1±0.6
Total P (μg/L)	105 ± 58	25 ± 17
Total C (mg/L)	13.5 ± 3.1	10.5 ± 1.3
HCO₃-C (mg/L)	6.7 ± 0.8	1.7 ± 0.5

Black and white waters differ significantly in their ionic composition, as shown in Table 1. Black waters are more acidic, resulting in an aluminum concentration greater than that of the more neutral white waters. The major difference is the concentrations of sodium, magnesium, calcium and potassium; these are very low in black waters. This has ecological implications. Some animals need more calcium than is available in blackwaters, so for example, snails, which need much calcium to build shells, are not abundant in blackwaters. The lack of dissolved ions in black waters results in a low conductivity, similar to that of rainwater.

Black and white waters differ in their planktonic fauna and flora. Tables 2 and 3 compare the number of planktonic animals caught in black and white water localities only a few meters apart. The black water was not as extreme an example as the Rio Negro system. However, it can be seen that the black water held greater numbers of rotifers but fewer crustaceans and mites. These crustaceans are important foods for larval fish. The zones where the two waters mix are attractive to ostracods and young fish. These mixing zones tend to have many animals. The abundance is shown in Table 3, which compares animals in 10 litres of water.^[5]

Table 2: Planktonic organisms collected in black (Japura) and white (Solimoes) waters.^[4]
Animal groups present	Black water	Mixed water	White water
Rotifera	284	23	0
Cladocera	5	29	43
Ostracoda	39	97	29
Calanoida	11	51	66
Cyclopoida	22	49	61
Chironomidae	0	3	3
Acari (mites)	0	0	2

Table 3: Number of planktonic organisms collected in 10 L of black, white and mixed waters.^[4]
	Black water		Mixed water		White water
Animal groups present	Open water	Forest	Open water	Forest	Open water	Forest
Volvocaceae	42		38
Rotifera	87	5	34
Cladocera	6		5		8	1
Ostracoda	2	11	3		7
Calanoida	23	3	10
Cyclopoida	5	27	19	1	13	1
Mysidacea		1
Diptera					1
Acari (mites)			1		1
Larval fish			1		1

Comparison between clear and black waters

Blackwater rivers resemble clearwater rivers (such as Tapajós, Tocantins, Xingu and some right tributaries of the Madeira) in having a low conductivity and relatively low levels of dissolved solids, but clearwater rivers have water that at most only is somewhat acidic (typical pH ~6.5)^[3] and very clear with a greenish color.^[6]