Winding rivers create ‘counterpoint bars’ regardless of the underlying geology

It is not uncommon for crescent-shaped sandy streets to stretch along the shorelines of meandering rivers. These sections usually appear on the inside of a river bend, where the bank turns around the sand spot and forms deposits known as’ ‘point bars’.

When they appear along an outer shore, which curves in the opposite direction, they form ‘counterpoint’ bars, which are usually interpreted by geoscientists as an anomaly: a sign that something – such as a piece of erosion-resistant rock – is interfering. with the river’s usual way of sediment deposition.

But according to research led by The University of Texas at Austin, counterpoint bars are not the peculiarities they often make up. In fact, it is a completely normal part of the winding process.

‘You do not need a resistant substrate, you can become beautiful [counter-point] without it, “said Zoltán Sylvester, a researcher at the UT’s Bureau of Economic Geology, who led the study.

The finding suggests that counterpoint bars – and the unique geology and ecology that go with them – are more common than previously thought. Aware of the fact, geoscientists can help to be on the lookout for countermeasures in geological formations that have been deposited by rivers in the past, and understand how this affects the flow of hydrocarbons and water flowing through them.

The research was published in the Geological Society of America Bulletin on March 12.

The co-authors are David Mohrig, a professor at the UT Jackson School of Geosciences; Paul Durkin, a professor at the University of Manitoba; and Stephen Hubbard, a professor at the University of Calgary.

Rivers are constantly on the move. For meandering rivers, it means cutting out new roads and reactivating old ones as they eventually snake across a landscape.

The researchers observed this behavior in an idealized computer model and in nature using satellite images of a section of the Mamoré River in Bolivia, which is known to change rapidly. The satellite photos captured how the river has changed over 32 years, from 1986-2018.

In both the model and the Mamoré, counterpoint bars appeared. The researchers found that the appearance was directly linked to short, high curvatures of curvature: small spines in the path of the river.

The researchers noted that these nails often form when the river’s course suddenly changes, such as when a new oxbow lake forms through the cut-off point, or after it is reconnected to an old oxbow lake.

But the sharp turns do not stay seated, but they pull in the downstream direction. And as they move downstream rapidly, it creates the conditions for sediment to accumulate around the bend as a counterpoint rod.

The study shows a number of cases that happened in the Mamoré. For example, in 2010 a sharp turn (turn 2 in the photo) forms when an oxbow lake is reconnected to a downstream section of the river. By 2018, the turn had moved about 1.5 miles downstream, with counter-point deposits along the coastline indicating its path.

Geomorphologists and engineers have long known that long-term changes along a river can be described by local and upstream curvature values ​​(places where the river appears to revolve around a small circle have high curvatures). In the study, the researchers used a formula that uses these curvature values ​​to determine the probability that a counterpoint bar will form at a particular location.

Sylvester said he was amazed at how well this formula – and the simplified models used in part to derive it – work to explain what is considered a complex phenomenon.

“Natural rivers, it’s actually not that far from what these really simple models predict,” Sylvester said.

This is not the first time that Sylvester’s research has revealed that river behavior can be controlled according to relatively simple rules. In 2019, he led a study published in Geology, which describes a direct link between belly sharpness and river migration.

Superficially, point rods and counterpoint rods look quite similar and mix frequently with each other. However, counterpoint bars are different environments: compared to point bars, they have finer sediments and lower topography, making them more susceptible to flooding and lake uptake. These properties create unique ecological niches along rivers. But they are also geologically important, with ancient counterpoint deposits deposited underground, affecting the flow of fluids such as water and oil and gas.

Mathieu Lapôtre, a geoscientist and assistant professor at Stanford University, said the recognition that counter-rods can easily form in meandering rivers – and that it has a formula for predicting where it will form – is an important advance.

‘Overall, the results of Sylvester et al. Important consequences for a series of scientific and engineering questions, ‘he said.