A computer-created graphic of a river meandering and associated sediment deposits. The lighter blue represents the river’s present circulation. The darker blue represents old locations of circulation that have actually been cut off due to the river’s meandering. The striped areas along the circulation courses represent sediment deposits in the type of point bars (red) and counter-point bars (blue). Credit: Sylvester et al.
It’s not unusual for crescent-shaped swaths of sand to dot the coastlines of meandering rivers. These swaths typically appear along the inner side of a river bend, where the bank twists around the sandy spot, forming deposits referred to as “point bars.”
When they appear along an external bank, which curves the opposite method, they form “counter-point” bars, which are typically analyzed by geoscientists as an abnormality: an indication that something – such as a spot of erosion-resistant rocks – is hindering the river’s normal way of sediment deposition.
But according to research study led by The University of Texas at Austin, counter-point bars are not the curiosity they’re typically constructed to be. In truth, they’re a completely regular part of the winding procedure.
“You don’t require a resistant substrate, you can get stunning [counter-point] bars without it,” stated Zoltan Sylvester, a research study researcher at UT’s Bureau of Economic Geology who led the research study.
The finding recommends that counter-point bars – and the special geology and ecology connected with them – are more typical than formerly believed. Building awareness around that truth can assist geoscientists watch for counter-point bars in geological developments transferred by rivers in the past, and comprehend how they might be affecting the circulation of hydrocarbons and water travelling through them.
The research study was released in the Geological Society of America Bulletin on March 12, 2021.
The co-authors are David Mohrig, a teacher at the UT Jackson School of Geosciences; Paul Durkin, a teacher at the University of Manitoba; and Stephen Hubbard, a teacher at the University of Calgary.
Rivers are continuously on the relocation. For meandering rivers, this indicates taking brand-new courses and reactivating old ones as they snake throughout a landscape in time.
The scientists observed this habits in both an idealized computer system design and in nature, utilizing satellite pictures of a stretch of Bolivia’s Mamoré River, which is understood for rapidly altering its course. The satellite pictures recorded how the river altered over 32 years, from 1986 – 2018.
Satellite pictures of the Mamoré River colored to highlight modifications in circulation course and sediment deposition as point bars (red) and counter-point bars (blue). From 2005-2010, the river (dark blue) goes through a neck cutoff (light blue). This modification in circulation course triggers little and extremely curved flexes to form (bends 1 and 2). Counter point bars form behind the bend 2 as it moves downstream. Credit: Sylvester et al.
In both the design and the Mamoré, counter-point bars appeared. The scientists discovered that the look was connected straight to short, high curvature flexes: little spikes in a river’s course.
The scientists observed that these spikes regularly form when the river’s course is suddenly altered, such as when a brand-new oxbow lake kinds through cutoff, or after reconnecting with an old oxbow lake.
But the sharp bends don’t sit tight, they begin moving in the downstream instructions. And as they quickly move downstream, they develop the conditions for sediment to build up around the bend as a counter-point bar.
The research study reveals a variety of circumstances of this taking place in the Mamoré. For example, in 2010, a sharp bend (bend 2 in the image) kinds when an ox-bow lake reconnects with a downstream part of the river. By 2018, the bend has actually moved about 1.5 miles downstream, with counter-point deposits along the coastline marking its course.
Geomorphologists and engineers understood for a long time that long-lasting modification along a river can be explained in regards to regional and upstream worths of curvature (locations where the river appears to twist around a little circle have high curvatures). In the research study, the scientists utilized a formula that utilizes these curvature worths to identify the possibility of a counter-point bar forming at a specific area.
Sylvester stated that he was amazed at how well this formula – and the streamlined designs utilized in part to obtain it – worked to discuss what was believed to be an intricate phenomenon.
“Natural rivers, they are actually not that far from what these really simple models predict,” Sylvester stated.
This is not the very first time that Sylvester’s research study has actually exposed that river habits can be governed by fairly basic guidelines. In 2019, he led a research study released in Geology that explained a direct relationship in between bend sharpness and river migration.
Superficially, point bars and counter-point bars look rather comparable and regularly mix into one another. But counter-point bars stand out environments: compared to point bars, they have finer sediments and lower topography, making them more vulnerable to flooding and hosting lakes. These qualities develop special environmental specific niches along rivers. But they are likewise geologically essential, with ancient counter-point bar deposits protected underground affecting the circulation of fluids, such as water and oil and gas.
Mathieu Lapôtre, a geoscientist and assistant teacher at Stanford University, stated that acknowledging that counter-point bars can easily form in meandering rivers – and having a formula for forecasting where they will form – is a substantial development.
“Altogether, the results of Sylvester et al. have important implications for a range of scientific and engineering questions,” he stated.
Reference: “Autogenic translation and counter point bar deposition in meandering rivers” by Z. Sylvester, P.R. Durkin, S.M. Hubbard and D. Mohrig, 12 March 2021, Geological Society of America Bulletin.
DOI: 10.1130/B35829.1
The research study was moneyed by the bureau’s Quantitative Clastics Laboratory research study consortium.
