So many of us are obsessed with the soft, acidic, botanical-influenced blackwater habitats that we study and work with that it's important to remember that there are many interesting rivers which which have clear water, mildly acid conditions, and some botanical influence. And some have intermediate characteristics which are even more interesting! Science since the 1950's has classified Amazonian rivers into three rather broad categories (whitewater, clearwater, and blackwater). And there are those which are influenced by multiple sources, and are sort of "mixed" waters.
The famous confluence of the Amazon and Rio Negro near Manaus, Brazil. (Pic by Mario Roberto Duran Ortiz, used under CC BY 3.0)
Fore reference, "Whitewater" rivers have a higher nutrient content than the blackwater and clearwater rivers, and the pH of water in turbid rivers is often near or above neutral (7.0). FYI don't confuse the local term "whitewater" used to describe these rivers to mean that they have lots of rapids- that's not what it's about, lol. It's really about the color and clarity in this instance.
"Clearwater" rivers typically have minimal loads of suspended sediments, low concentration of major nutrients, and a chemical composition similar to rainwater in many respects. The majority of clearwater rivers and streams in the central and eastern Amazon Basin are known to be slightly acidic, with pH values varying between approximately 6.0 and 6.8. However, these values may be higher in the western tributaries closer to the Andes.
And of course, our much-loved "blackwater" rivers need almost no description at this point, but for sake of completeness, we'll simply say that this term describes tea-like water in which plant and botanical compounds are not completely decomposed, or as scientists would state, "Water in which rate of carbon fixation (photosynthesis) and its partial decay into soluble organic acids exceeds its rate of complete decay to carbon dioxide."
And, as we already know, the substantial concentrations of organics in blackwater rivers renders them quite acidic, ranging in pH from 3.8 to 5.4.
And then there are those rivers which, although classified primarily into one of the descriptors, are influenced by varying sources to the point where one might even dare call them "mixed" waters...
One such river is the Tapajós, and the other minor tributaries in the Tapajós River Basin. Standing in sharp contrast to our beloved Amazonian blackwaters, which, as we know, have pH values well below 5.0, very low electrical conductivity, and general lack of carbonates, the Tapajós may be loosely characterized as a "clear water" habitat, with a greater electrical conductivity, pH around 6.5 or above, and more richness of carbonates. The lower Tapajós demonstrates really high transparency because there is not a major current, and the sediments brought in have already been deposited.
Now, as we alluded to above, scientists who research this stuff will tell you that many rivers and streams of Amazonia have to be considered as "mixed waters" of intermediate, resulting from the influence of tributaries with different chemical compositions, and the Tapajós definitely falls into this category. Differences in nutrient levels in these rivers and the tributaries which feed them also impact fish species composition, richness, and growth. It's a rich area for fishes, with about 325 fish species are known from the Tapajós River basin, including 65 endemics.
The Tapajós in the "grand scheme" of Amazonia (Map by mussier, used under CC BY-S.A. 3.0)
One of the major influences on the Tapajós is a what is known as a "Ria Lake", formed by the eroding of the river valley due to aggradation processes that occurred during the Holocene Period. It's a large mouth lake, surrounded by relatively high cliffs and low-lying beaches. The beaches are inundated for around six months each year and support sandy-flooded-forest species of plants in large numbers. This obviously has some influence on the composition of the water during the inundation. In many regions of the Tapajós, turbid water, clear water and black water actually exist in really close proximity. The blackwater rivers nearby drain sandy soils and flow into the clearwater Tapajós near its confluence with the Amazon River, so you can see this whole "mixing" thing in action!
And, like so many rivers and waterways in Amazonia, seasonal rains have a huge impact. Because of it's sheer geographic size and range, different areas of the Tapajós experience rainy seasons at different times. The rainy season in the upper Tapajós Basin begins in late September, while in the lower basin, the rain begins in late December or January. The peak of the annual inundation in the middle and upper Tapajós Basin is typically around March. In the areas nearest to the Tapajós River mouth, the months with the highest water levels are normally May or June, because the water levels in the lower river are controlled by none other than the Amazon River, which has it's own "schedule" to keep!
So much influence from so many different sources. Rivers and streams in general are impacted by many other habitats, and this is what creates these "mixed waters" that we're talking about. And, as hobbyists, such variations and influences create unique opportunities to replicate some interesting aquatic habitats in our aquariums!
(Amazon and some tributaries meet near Manaus. Pic by Neil Palmler, used under CC BY-S.A. 2.0)
Now, in our aquarium practice, many of us would be hesitant to explore pH levels much below the "mid fives", although there are a number of intrepid aquarists who do experimental work with very low pH waters with certain fishes (i.e.; wild Angelfish). And, there are a fair number of hobbyists out there who like the idea of utilizing botanical materials and leaves in their tanks, but are simply not big fans of the tinted water. Replicating a habitat like the Tapajós or it's minor tributaries might be a nice compromise!
This necessarily general, short visit to the concept of mixed waters will hopefully stimulate you to do some research on your own, and to consider that there are environments out there for just about everything you'd want to play with, aquarium-wise! Aquascaping, selection of substrate and other materials, as well as the choice of aquatic plants- not to mention, our fish selections- are all influenced by the composition of the water, and we should take these factors into consideration when planning our aquariums.
Now more than ever, we have lots of good scientific data on many of the areas from which our fishes come from. We have not just water chemistry information- we have information about the influencing soils, climate, plants, and associated fauna to work with. Yes, you can be as hardcore biotope-driven as you want, or can simply take some major cues and go from there. The choice is yours, and you can enjoy attempting to replicate the aquatic habitat of your choice to as great an extent as you'd like.
It's an amazing time to be an aquarist, and there has never been a better time to take another look at the natural world when contemplating our next aquarium.
And it all starts with looking at the magic that happens when the waters mix.
(The Madeira River and the Aripuanã River mix. Pic by Neil Palmler, used under CC BY-SA 2.0)
Until next time...
Stay curious. Stay resourceful. Stay inquisitive. Stay creative...
And Stay Wet.