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Rio Negro Basin, Brazil

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Educated by Nature

Brazil, Amazonas

The Rio Negro is the largest blackwater tributary of the Amazon River. As the name suggests, the Rio Negro means “black river” although it is not as black as imagined and even looks like strong tea. The black color comes from organic acids that are not completely broken down by the vegetation around the river.

In Colombia, the Rio Negro River is also known as the Río Guainía, the river flows into the Amazon River in region of Manaus.

Submitted by
Mohammad Arief Rachman Hakim
Approved by
Roberto E. Reis & Pablo C. Lehmann
Geographical region
South America
Drainage Basin
Rio Amazonas
River catchment
Rio Negro
Water body type
Igarapè
Water body name
Nameless affluent
Water body part
Marsh land
Water body course
Lower course
Water body: tributary of
Rio
Tributary name
Rio Negro

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Water Chemistry

Water information

Water type
fresh-water
Water color
Black water
Water transparency
Medium
Concentration of sediments
Medium
Water temperature
29-30 °C
Water flow/curent
Slow

Chemical parameters

pH
Conductivity
8
GH
dGH
KH
dKH
Dissolved Oxygen

Substrate in nature

Sand
White
Pebble/Gravel
Orange
Stone
Brown
Stone form
Irregular
Silt/Mud
None
Leaves
Many
Driftwood
Many
Submerged terrestrial vegetation
Yes

Aquatic Biotope

Date of collecting
15 February, 2024
Collecting area
Flooded area
Water depth
1,0m
Air temperature
Sunlight
Filtered/dappled sun

Environment

Environment
Affected by human activity
Affected by human activity
Deforestation
Agriculture
Advanced
Deforestation
Initial
Surrounding area

This ecoregion includes the Rio Negro drainage basin from Manaus in the east to the slopes of the eastern cordillera of the Andes in Colombia. It includes the lower Rio Branco downstream from Caracaraí and the lower Rio Jauaperi basin. It is bounded in the north by the mountainous divide between the Negro and the Orinoco and upper Rio Branco drainage. The southern limit is formed by the low-lying divide between the Rio Negro tributaries and the Rio Solimões drainage.

Underwater landscape

Low gradient rivers and streams run through sedimentary soils formed mainly by podsols subjected to seasonal flooding. The Rio Negro’s water is extremely poor in mineral content, with conductivity as low as 8µS, and is extremely acidic, with pH ranging 2.9-4.2.

The largest blackwater river in the world, the Rio Negro provides roughly 14% of the average annual discharge into the Amazon, second only to the Madeira. Its main tributary, the Rio Branco, is by contrast a whitewater river. Although not as turbid and the Amazon or Madeira, it is muddy during the flood season. The sediments are visible 200km downstream of the confluence with the Negro.

The headwaters of the Negro originate in the foothills of the Andes. In the upper reaches the flood season occurs between May and September, peaking in July. Water level fluctuations in the lower reaches are dictated more by the Amazon, and occur earlier.

Here, the flood season lasts from February through July, with peak water levels in June. The average annual river fluctuations range between 4-5m in the upper reaches and 10m in the lower reaches. It is estimated that an area of 30.000km² within the Negro basin is seasonally flooded between 4-8 months of the year. The largest floodplains occur along right-bank tributaries, as well as among the network of islands along the middle and lower Rio Negro.

There are many insular environments along the main stem of the Rio Negro, including more than 600 islands in the lower and mid-section of the river. Along the main stem and tributaries are vast fringing floodplains covered by flooded forests and inundated campinas and caatingas (campinarana), as well as many floodplain lakes and oxbow lakes along the channels. During the dry season vast sand beaches are found along the entire extent of the rivers. River bottoms are rocky with gravel and large boulders. Rocky outcrops and cataracts reveal evidence of the Guiana shield at points along the Rio Negro’s middle reaches.

Threats to ecology

Ecological phenomena include large migrations of doradid catfishes, characids such as Brycon, and Prochilodontids of the genus Semaprochilodus. The jaraqui (Semaprochilodus insignis), for example, migrates from blackwater to whitewater rivers to spawn. There are also unique assemblages of species on leave-litter deposits, and many miniaturized forms.

Riparian zone

Trees near the aquatic habitat
Many -