The Confluence of Rivers

January 13, 2014

The Confluence of Rivers

Photographer: Dave Lynch; Dave’s Web site
Summary Author: Dave Lynch

When two rivers flow together, there’s usually a difference in water color that persists downstream of their confluence until the color of the large river comes to dominate. The color of each river is determined by many things: type and amount of vegetation in the watershed, geological properties, dissolved chemicals, suspended sediments and biologic content -- usually algae.

Confluenceofrivers1A great example of this occurs near Manaus, Brazil where the Negro River flows into the Amazon River (Rio Solimoes). The two rivers don’t mix immediately. Instead water from the Negro, which is actually dark blood red, flows lazily alongside the yellowish-brown Solimoes water, with a sharp boundary between them. Only many miles downstream do the two waters gradually merge and the mixing becomes complete. The much larger flow volume from the tan Solimoes dominates the Amazon River all the way to its mouth, a thousand miles (1,600 km) away at the Atlantic Ocean.

The Rio Negro, a so-called blackwater river, originates in low, tropical rainforest while the Solimoes starts high in the Andes Mountains of Peru and Ecuador. Tannin and humic acid from the decay of dead vegetation are the primary sources of the Negro’s color. Top photo taken in 1982 by the author. Bottom photo adapted from Google Earth.