A blackwater river is 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 coffee. Most major blackwater rivers are in the Amazon River system 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. The unique conditions lead to flora and fauna that differs from that in whitewater. Where blackwater and whitewater combines is attractive to a diverse group of organisms.
Comparison between white and black waters
|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 aluminium 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.
|Animal groups present||Black water||Mixed water||White water|
|Black water||Mixed water||White water|
|Animal groups present||Open water||Forest||Open water||Forest||Open water||Forest|
Blackwater rivers of the world
- Apaporis River: A tributary of the Yapura River.
- Arapiuns River: A tributary of the Tapajós River.
- Coari River
- Mirití-Paraná River
- Rio Negro: The largest blackwater river in the world; one of the largest Amazonian tributaries.
- Piorini River
- Tahuayo River
- Tefé River
- Uatamã River
- Urubu River
- Vaupés River
- Atabapo: from the Guiana Highlands of Venezuela west into the Orinoco
- Caroní River: from the Guiana Highlands of Venezuela north into the Orinoco.
- Caura River (Venezuela): from the Guiana Highlands of Venezuela north into the Orinoco
- Inírida: from Colombia northeast into the Guaviare River which flows into the Orinoco
- Ventuari: from eastern Venezuela southwest into the Orinoco
- Vichada: from Colombia east into the Orinoco
- Tomo: from Colombia east into the Orinoco
- Tuparro: from Colombia east into the Orinoco
Southern United States
- Ashepoo River: Along with the Edisto and the Combahee Rivers in South Carolina makes up the ACE Basin National Wildlife Refuge.
- Big Cypress, Black Cypress, and Little Cypress and the small rivers in the watershed of Caddo Lake in Texas and Louisiana.
- Blackwater River: a tributary of the Chowan River in Virginia.
- Blackwater River: a major river in the western Florida panhandle.
- Black River: a tributary of the Pee Dee River in North and South Carolina.
- Cape Fear River, North Carolina: flows into the Atlantic Ocean.
- Cashie River, North Carolina: flows into Albemarle Sound.
- Caloosahatchee River, Florida: flows west from Lake Okeechobee to the Gulf of Mexico.
- Chowan River, North Carolina: flows into Albemarle Sound.
- Edisto River, South Carolina: flows into the Atlantic Ocean; the longest undammed, unleveed blackwater river in North America.
- Econlockhatchee River, a tributary of the St. Johns River in Central Florida.
- Four Hole Swamp, a tributary of the Edisto River in South Carolina.
- Little Manatee River, Florida: flows into Tampa Bay.
- Great Coharie Creek, North Carolina: flows into the Black River.
- Little Pee Dee River, South Carolina: flows into the Pee Dee River.
- Lynches River, South Carolina: flows into the Pee Dee River.
- Lumber/Drowning Creek: located in North and South Carolina. Part of Lumber River State Park
- Ogeechee River: A 245-mile river in eastern Georgia that passes to the south of the city of Savannah and enters the Atlantic Ocean at Ossabaw Sound.
- Pithlachascotee River: A small river in central Florida.
- Santa Fe River: A river in northern Florida.
- Satilla River: A river in southeast Georgia that flows through the city of Waycross and empties into the Atlantic Ocean near Cumberland Island.
- Scuppernong River: A small river in Washington and Tyrrell Counties in eastern North Carolina at Pettigrew State Park.
- St. Marys River (Georgia): 
- St. Johns River: The largest river in Florida. Flows north through Jacksonville and into the Atlantic.
- Suwannee River: A large river in southern Georgia and northern Florida flowing into the Gulf of Mexico.
- Upper Little River, North Carolina: flows into the Cape Fear River.
- Blackwater River, West Virginia: Located in the Blackwater Canyon in Tucker County. Blackwater Falls a five-story waterfall is located along this river with rapids ranging from Class III-V+
- Waccamaw River, North and South Carolina: flows into the Atlantic Ocean.
- White Oak River, North Carolina: flows into the Atlantic Ocean.
- Pocomoke River, Maryland: flows into the Pocomoke Sound / Chesapeake Bay.
Northern United States
- Black River, New York: A river starting in the western Adirondacks that flows into Lake Ontario.
- Ocqueoc River, Michigan: A river in the northern Lower Peninsula that flows in Lake Huron
- Tahquamenon River, Michigan: A river in the Upper Peninsula that flows into Lake Superior.
- Tuckahoe River, New Jersey: A short river in southern New Jersey that flows into Great Egg Harbor.
- Gooseberry River: A river in northern Minnesota that leaches from peat bogs and pine forests.
- Gordon River, Tasmania: A river rising in the center of the island and flowing westward, emptying into Macquarie Harbour on the West Coast.
- Pieman River, Tasmania: A river on the West Coast of Tasmania which leaches from rainforests and heathlands.
- Davey River, Tasmania: A relatively small river in the southwest corner of the island which runs through extensive heathlands, peat bogs and button grass plains.
Images of blackwater rivers
- Janzen, D H (July 1974). "Tropical Blackwater Rivers, Animals, and Mast Fruiting by the Dipterocarpaceae". Biotropica 6 (2): pp. 69–103. doi:10.2307/2989823. JSTOR 2989823.
- Sioli, Harald (1975). "Tropical rivers as expressions of their terrestrial environments". Tropical Ecological Systems/Trends in Terrestrial and Aquatic Research (New York City: Springer-Verlag): pp. 275–288.
- J S B, Ribeiro; A J Darwich (1993). "Phytoplanktonic primary production of a fluvial island lake in the Central Amazon (Lago do Rei, Ilha do Careiro)". Amazoniana (Kiel) 12 (3-4): pp. 365–383.
- "Comparison between white and black waters". Amazonian Fishes and their Habitats. Pisces Conservation Ltd. Retrieved 2006-05-21.