Meandering rivers create ‘counterpoint beams’ regardless of the underlying geology

It is not uncommon for crescent-shaped sandstrips to dot the banks of meandering rivers. These strips usually appear along the inside of a river bend, where the bank wraps around the expanse of sand and forms deposits known as “pointed bars”.

When they appear along an outer bank, curving in the opposite direction, they form ‘counterpoint’ beams, usually interpreted by geoscientists as an anomaly: a sign that something – such as a patch of erosion-resistant rock – is interfering with the usual way sedimentation of the river.

But according to research led by the University of Texas at Austin, counterpoint rods aren’t the quirks they are often referred to. In fact, they are a completely normal part of the meandering process.

“You don’t need a resistant substrate, you can grow beautiful [counter-point] bars without, ”says Zoltán Sylvester, a research scientist at the UT’s Bureau of Economic Geology who led the study.

The finding suggests that counterpoint bars – and the unique geology and ecology associated with them – are more common than previously thought. Awareness of that fact can help geoscientists look for counterpoint rods in geologic formations deposited by rivers in the past and understand how they affect the flow of hydrocarbons and water flowing through them.

The research is 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 in constant motion. For meandering rivers, this means carving out new paths and reactivating old ones as they meander through a landscape over time.

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

Counterpoint beams appeared in both the model and the Mamoré. The researchers found that appearance was directly related to short, high curves – tiny peaks in the path of a river.

The researchers noted that these peaks are often formed when the course of the river is changed abruptly, such as when a new oxbow lake is formed by truncation or after reconnecting with an old oxbow lake.

But the sharp turns don’t stay in place, they pull downstream. And as they move rapidly downstream, they create the conditions for sediment to accumulate around the bend like a counterpoint rod.

The study shows a number of examples of this happening in the Mamoré. For example, in 2010, a sharp bend (bend 2 in the image) forms when an ox-bow lake reconnects to a downstream part of the river. In 2018, the bend moved about 2.5 miles downstream, with counterpoint deposits along the shoreline marking the path.

Geomorphologists and engineers had known for some time that long-term changes along a river can be described in terms of local and upstream curvature values ​​(places where the river appears to wrap around a small circle have high curves). In the study, the researchers used a formula that uses these curvature values ​​to determine the probability of a counterpoint bar forming at a particular location.

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

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

It is not the first time that Sylvester’s research has shown that river behavior can be controlled by relatively simple rules. In 2019 he led a study published in Geology describing a direct relationship between bend sharpness and river migration.

On the surface, point beams and counterpoint beams look quite similar and often merge into each other. But counterpoint bars are different environments: Compared to point bars, they have finer sediments and lower topography, making them more prone to flooding and lakes. These features create unique ecological niches along rivers. But they are also geologically important, with ancient, underground-preserved counter-pointed rods affecting the flow of liquids, such as water and oil and gas.

Mathieu Lapôtre, a geoscientist and assistant professor at Stanford University, said recognizing that counterpoint rods can be easily formed in meandering rivers – and having a formula to predict where they will form – is a significant advance.

“All in all, the results of Sylvester et al. Have important implications for a range of scientific and technical questions,” he said.