The emergence of land plants around 450 million years ago significantly influenced Earth’s geological processes, particularly weathering and erosion. While earlier scientists speculated that plants might have caused straight rivers to meander—due to a lack of evidence showing naturally meandering rivers—recent studies have challenged this perspective. Researchers now propose that vegetation altered rivers without leading to their meandering.
To investigate how vegetation affected ancient rivers, researchers analyzed 49 modern meandering rivers. They categorized these rivers into three distinct types: vegetated, non-vegetated, and semi-vegetated. Their study revealed 18 vegetated rivers in South America, 24 non-vegetated rivers in the western United States, and seven semi-vegetated rivers in areas of China and the eastern United States.
To evaluate the influence of vegetation on curvature, the researchers quantified the degree of each river’s bends. They determined the curves by using the opposite bank of each bend to establish a center point and then drew equidistant lines along the river’s path. This method enabled them to calculate the angle between the curves and the central point, termed the transition angle. This angle illustrates the river bend’s relationship to its downstream direction, providing insights into how vegetation impacts the vertical or horizontal development of the river and the steepness of its bends.
The researchers analyzed the migration angles of each river to assess differences in bends between vegetated and non-vegetated systems. Their findings indicated that vegetated rivers tended to deposit sediment within bends, promoting wider and more horizontally developed curves over time. In contrast, non-vegetated rivers deposited sediment downstream, resulting in fewer bends and more variation in bend widths.
A key question remained: were these differences mainly due to vegetation or influenced by other factors? To address this, the researchers explored three additional variables. The first was the natural variation in water flow across river systems, known as flow rate fluctuation. They found that during storms, flow fluctuations shifted bends downstream in non-vegetated rivers but had no effect on vegetated ones, suggesting that while flow fluctuations can influence vegetation, they don’t necessarily lead to downstream migration.
The second variable analyzed was the river’s sediment capacity or sediment flux. The study concluded that rivers carrying more sediment did erode their banks more, influencing bends. However, rivers with similar vegetation cover and higher sediment levels demonstrated statistically similar tortuosity angles, indicating that sediment flow alone doesn’t drive bend development; instead, that changes are contingent upon vegetation cover.
The final variable studied was the strength of riverbanks. Researchers noted differences between strong banks made of rocks and compact sediments versus weak banks of loose sediment. Findings showed no significant differences in bends among rivers with varying bank strengths but similar vegetation, ruling out bank strength as a primary factor influencing bend movement.
Among the four examined variables—flow variability, sediment flux, bank strength, and vegetation cover—vegetation cover emerged as having the most substantial impact on the characteristics of meandering rivers. The researchers speculated that while winding rivers may have existed prior to the advent of plants, their appearance would have been markedly different. Like today’s non-vegetated rivers, ancient meandering rivers likely had gentler bends. As plants evolved and established themselves along riverbanks, the bends transformed to resemble modern vegetated rivers. This understanding could shed light on life on Earth 450 million years before the evolution of land plants.
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Source: sciworthy.com
