When did the Amazon waterfalls appear? – 09/11/2023 – Fundamental Science

Anyone who has been to the Amazon has certainly come across exuberant waterfalls, spread throughout the region. In large rivers, they are points of hydro energy production; In the igarapés — the name given to the smaller rivers there, from the Tupi “canoe path” —, they attract tourists and locals for recreational purposes. They can also delimit the geographic distribution of aquatic species, constituting biogeographic barriers for some species. But what is its existence due to? When did they appear? The answers to these questions could help unravel the evolutionary history of the Amazon River.

Waterfalls are vertical breaks, or steps, in riverbeds. In the case of the Amazon, they reach 10 meters or more. To understand how a waterfall of this size can arise, imagine an unmaintained swimming pool: as it dries, the greater the drop between its edge and the water. Now, imagine a gentle stream that flows uninterruptedly until its confluence with the main river; If the latter, for any reason, suffers a drop of tens of meters, a waterfall forms between it and the stream.

When the level of the main rivers — in the hypothetical case, the pool — is lowered over geological time, it is said that there has been a fall in the base level. Just as an empty swimming pool reveals its tiles, a river whose bed has been lowered reveals its rocky substrate. Have you ever noticed how there are exposed rocks in the beds of waterfalls and rapids?

The rocky step in the riverbed is an increase in slope, a “corner” that is then eroded, leading to its retreat. Therefore, although they appear not to be moving, the waterfalls recede over geological time at slow speeds much less than one millimeter per year.

Imagine, therefore, that a single waterfall generated at the mouth of a river can multiply into dozens of others as it recedes upstream. After millions of years, the process results in dozens of waterfalls spread across the landscape. More fascinating than this is to contemplate that, using equations that model erosion in the rocky bed of a river, it is possible to simulate the migration of waterfalls from their origin and, thus, get very close to the current geographic distribution. In this way, the time since the first drop in the base level can be quantified.

It is likely that dozens of waterfalls in the Amazon resulted from a single drop in base level in the geological past. Like a domino effect, a drop in the base level of the Amazon River would generate steps in all of its tributaries. Then, these steps would propagate upstream, transmitting the cascades to the streams of their tributaries and from there to the streams, and so on. Proof of this is that dozens of waterfalls in the northern channel of the lower Amazon River are at the same elevation despite being tens to hundreds of kilometers away from each other, which occurs due to a process of falling the base level in the main river, that is, they have a single origin.

But what would be capable of generating a drop in the base level in Amazonas? There are some plausible mechanisms, such as drops in global sea level during glacial periods, perhaps the most popular hypothesis among geoscientists. Lots of ice in glaciers, less water in the oceans.

Thus, a glacial period could lower the global sea level and, consequently, the Amazon River — the waterfalls would then have started with the glaciations in the last 2.6 million years, reaching the last one, approximately 20 thousand years ago. But would such a phenomenon be powerful enough to cause waterfalls in the stream beds? To date, few studies have put this question to the test through quantification based on erosion processes.

Another plausible mechanism would be associated with the geological evolution of the basin. Before its current configuration, the Amazon basin was made up of two parts, one that flowed towards the Atlantic through the current mouth, and another, much larger, that also flowed towards the Atlantic through the mouth of the current Orinoco River, in Venezuela. The divide between these proto-basins was close to Manaus. However, knowing when it was ruptured to join the two basins is still a controversial issue. This union would have multiplied the water flow of the eastern Amazon and consequently would have promoted the lowering of its bed. In this case, the fall in the base level in the eastern Amazon would occur at some point in the last nine million years.

These are just two hypotheses among several more complex ones. The conclusion is that, although, most of the time, geologists look for geochronological evidence in rocks and sediments, we can also use landscapes, which hold geological memories, and the physical principles that govern them. In the case of waterfalls in the Amazon it is no different. It remains to be seen when and why they were formed, and the answers could be the key to unraveling part of the geological and even biological evolution of the largest river basin on the planet.

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Pedro Val is a geologist and professor at Queens College, City University of New York.

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