Posted by: Rob Viens | October 13, 2013

Meandering Around the Parana

Much like Darwin, I spent most of the last week lying low (at least from writing). Though unlike him, I was not in the city of Santa Fe in northern Argentina. In his one entry for the week Darwin describes the region where he had been passing time:

“The Bajada itself is quiet town; about as large as St Fe or St Nicholas; it contained in 1825, 6000 inhabitants. — The whole province only contains 30,000. — Yet here there are representatives, ministers, standing army, governors &c &c. Few, as they are, none have suffered more from desperate & bloody revolutions. — In some future day however this will be one of the finest provinces. — As its name expresses, it is surrounded on every side by the magnificent rivers, the Parana & Uruguay. — the land is most fertile. — Here there is no fear of the Indians; an immense advantage over their neighbours; to the North of St Fe, there is not a single Estancia on the West of the Parana; & we have seen that the road is not safe between the Capital & Corunda.” (Oct 6-11)

He had planned to use the time to explore the local geology. However, as much as he liked exploring the rocks and fossils, Darwin was not too impressed with what the region had to offer in that regard. He writes (disappointedly):

“By the indolence of the master & from had weather I was delayed five days. — The time passed pleasantly & was enabled to see the geology of the surrounding district. — And this possessed no common interest.” (Oct 6-11)

There may not have been a lot of rock exposed in the region, and the fossils didn’t live up to earlier finds further down stream, but the river itself provided a great example of how geologic processes shape the landscape. If Darwin only had an airplane it would have been just as fascinating as the rocks! (Well, almost…)

We tend to think about geology as the study or rocks and minerals, earthquakes and volcanoes, and fossils and deep time. But it is also the study of how rivers, glaciers, landslides, etc. shape the surface.  The study of these processes falls into the general field of geomorphology. The lower reaches of the Parana River, as well as the Rio Negro that Darwin explored a couple of months ago, illustrate some great examples of fluvial (river) geomorphology.

For example, rivers are very dynamic systems, with channel that are constantly changing location.  (This is why it is so hard to “control” a river.)  In their lower reaches, where the land is relatively flat, the river tends to put its energy into cutting away at its banks. (Further upstream, energy is often directed into downcutting the channel, creating canyons and gorges.).  As the water passes a bend in the river it has to speed up along the outer bank (compared to the inside of the bend) to cover a longer distance.  Picture the last time you stood on a river bank or floated down a river on a raft and you might be able to picture what I mean.  The inner bend is always “beach-like” (forming a feature called a point bar) – often made of sand or gravel.  The outer bend often consists of a deep pool at the base of a steep “cut bank”. This physical difference between the inside and outside of the bend is due to the difference in water speed. The faster water moving along the outside bend has more energy and cuts away at the bank.  The slower water on the inside bend deposits sediment creating the gravelly point bar.

“Physics” of a bend in the river showing the water eroding cut banks and depositing point bars:

cut banks and point bars

Now if you take this to its logical conclusion – the river cutting away at the outside of the bend and depositing sediment on the inside – it makes sense that the river channel would move over time. Specifically the bend would get more extreme.  And in fact, this movement of the channel will continue until you get a large horse-shaped bend in the river that practically wraps back on itself.  Eventually these convoluted bends will reconnect, and the river will abandon the longer bending channel, leaving behind an oxbow lake (named so because it looks like the plow yoke used for oxen). These lakes eventually fill up with sediment, leaving meander scars – hints of where the old river used to flow.  And the process starts all over again.

Meander scar and oxbow lake formation:

meander scar formation

If you do this for long enough (and you have plenty of time in geology), what you get are deeply scarred landscapes that show an ever-moving river channel.  A world-class example of this can be seen on the Rio Negro (where Darwin was a coupe months ago), and to some degree on the Rio Parana.

Classic NASA image of Rio Negro meander scars:

meander scars on Rio Negro

Meander scars on the Rio Negro from Google Earth:

meander scars on Rio Negro

Meandering path of the Rio Parana from Google Earth:

meanders on Rio Parana

The week was not all bad for Darwin as he waited for transport back down the meandering river.  For one thing, the birds and flowers brought back memories of one of the most magical places Darwin had ever been – the Brazilian Rainforest.  He wrote:

“My usual walk during these days was to the cliffs on the Parana to admire the view of the river & pick up fossil shells. — Amongst the fallen masses of rock, vegetation was very luxuriant; there were many beautiful flowers, around which humming birds were hovering. — I could almost fancy that I was transported to that earthly paradise, Brazil.” (Oct 6-11)

The wait would soon be over and Darwin would be heading back downstream to the chaos of Buenos Aires. (RJV)

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