Some coastal and inland drainage areas are dominated by wetlands, with high organic and tannic content, and the waters may appear black, and sometimes referred to as black waters black water streams or rivers, as opposed to brown water streams that have brown appearance during storm events from soil and bank erosion. If this occurs only in rainy season, perhaps this is contributions from isolated wetland outflow or overflow.

In general, the water liquid is not colourless. It absorbs certain wavelengths which normally gives it a pale blue colour. This crystalline colour is rarely observable because running water charges quickly into organic matter, into suspended matter, into plants too; and in addition into dissolved mineral salts. It should be noted that the different compounds of iron tints water yellow or red, and calcium carbonate green. For the black colour, it might be sulphides.

Finally, the geology and land use of the watershed individualize the colour of each water body. But nowadays, pollutions due to human activities, especially eutrophication, might have a decisive role on the colour of the waters. Various elements on how to address this problem of eutrophication from an economic point of view are presented in one of my open access publications on RG.

In the Amazon Basin some 14% of the water flow is black Rio Negro.

This color as alluded to by a previous contributor this darkness is associated with tannins which are the constitutents that color tea.

The dark colors can also be contributed by erosion this is not the case with Rio Negro or the black river of the Amazon Basin however which are some of the cleanest fresh water rivers globally.

Humic acid are black or dark brown and fluvic acids are golden. Solublized old clays will give a red or yellow appearance also.

In Bangladesh, the river Buriganga always seems black and it is due to pollutant of different org dye from textile industry. Also the river Turag near Tangi remains black due to some industrial fluid discharged from near by areas and also due to sewage discharged from areas.

Dear Colleagues,

  Thank you for your answers and attention. I have noticed the blackness of Shatt Al-Arab waters and this didn't happen before.

Accept my greetings