A view of seasonal flow in Shoshone Creek – an unvegetated meandering stream in Nevada, USA. Credit: M. Hasson and M. Lapôtre
New research confirms that the rise of land-based plant life changed the way rivers move, but not in the way geologists have thought, and taught, for decades.
The new study in the journal Science has discovered a new way in which floodplain vegetation – the plant life which grows in flat land next to a river – controls river dynamics.
Today, rivers on Earth come in 2 types: braided and meandering.
Meandering ones form single channels that curve back and forth across the landscape. Well known examples include the Rio Grande in the US and Africa’s River Nile.
Braided ones, like the Brahmaputra which flows through Tibet, India, and Bangladesh, widen and divide into many interwoven channels separated by islands of sediment.
The findings of a new study in the journal Science cast into doubt the widely accepted idea that braided rivers dominated the planet’s surface for its first 4 billion years, before the widespread appearance of vascular plants with root systems.
“With our study, we’re pushing back on the widely accepted story of what landscapes looked like when plant life first evolved on land,” says lead author Michael Hasson, a PhD student at Stanford University’s Doerr School of Sustainability in the US. “We’re rewriting the story of the intertwined relationship between plants and rivers, which is a significant revision to our understanding of the history of the Earth.”
Hasson and colleagues studied satellite images of about 4,500 bends in 49 meandering rivers to better understand the effect of plants on river channels. Half of the bends lacked vegetation growing in the surrounding floodplain, the others were densely or partially vegetated.
The researchers homed in on a feature of meandering rivers called point bars – the sandy landforms that develop on the insides of their bends.
“As a river flows, sand and gravel on the streambed are carried downstream. These sediments, which are collected in bars (elevated areas of sediment) and other deposits, move around and create features that record the directions of a river’s flow,” Jim Pizzuto from the University of Delaware’s Department of Earth Sciences in the US, writes in a related Perspective.
“Braided, multi-channel rivers tend to flow straight down their valleys. By contrast, winding, single-channel meandering rivers have variable flow directions that reflect their sinuous courses.”
Unlike the sediment islands that form in the middle of braided rivers, point bars tend to migrate laterally – to either side of the water’s flow – which contributes to a meandering river’s curving path.
Over time, these river deposits transform into sedimentary rocks which preserve information about the flow direction and character of ancient rivers.
A drone view of the active channel and floodplain of Shoshone Creek in Nevada, USA. The active river channel is moving through sediments it previously deposited. Former channel boundaries visible at the surface record the overall downstream migration of river bends, as Hasson et al. showed typically occurs in meandering rivers with bare, unvegetated banks. Credit: M. Hasson and M. Lapôtre
Previous analyses of these ancient rocks suggested that rivers in the early Palaeozoic Era – more than 419mya, before land plants became widespread – primarily ran straight downstream.
“These studies, coupled with observations of vegetation inhibiting the formation of multi-channelled braided rivers, have led to a proposal that single-channel meandering rivers only became widespread on Earth after land plants developed in the Palaeozoic Era,” writes Pizzuto.
“This interpretation has been challenged by the discovery of well-developed meanders (or bends) in unvegetated arid regions on Earth, and the presence of unvegetated meandering channels on Mars and Titan.”
The new study provides further evidence against the long-held theory that vegetation is needed for river meandering.
The researchers found that point bars in meandering rivers with vegetated floodplains tend to migrate laterally (side-to-side) across valley. But in the absence of plants, point bars instead migrate downstream the same way that mid-channel bars do in braided rivers.
“If one were to use the same criterion geologists use in ancient rocks on modern rivers, meandering rivers would be miscategorised as braided rivers,” says Mathieu Lapôtre, senior author and assistant professor of earth and planetary sciences at Stanford.
The findings have implications for understanding river dynamics on other planets and in Earth’s early history.
“The growing field of river restoration and ecological engineering could also benefit because restoring rivers often involves planting vegetation around artificially constructed river meanders,” adds Pizzuto.
“The detailed interactions between river meanders and vegetation could help improve restoration designs to create self-sustaining environments with improved water quality and a flourishing animal and plant life surrounding a river.”