The evolution of land plants about 450 million years ago altered many of Earth’s geologic processes, like weathering and erosion. Due to the lack of evidence for meandering rivers before then, past scientists hypothesized that plants could have caused straight rivers to meander. However, in recent decades, researchers have challenged this idea. They’ve suggested that plants could have changed rivers without causing them to meander.
To understand how vegetation changed rivers in the past, researchers recently studied 49 modern meandering rivers. They sorted these rivers into 3 categories – vegetated, unvegetated, and semi-vegetated – by analyzing color images taken of them from the air. They identified 18 vegetated rivers located in South America, 24 unvegetated rivers in the western United States, and 7 semi-vegetated rivers in China and the Eastern United States.
To examine the impact of plants on these rivers, the researchers quantified how much each river channel curves, known as its sinuosity. They used opposite banks of each river bend to find its center point, then, using digital maps, drew a line along the river’s trajectory at an equal distance between the bend center points. They used this line to calculate the angle between the river’s curve and the center point. This angle, called the migration angle, shows how a river bend relates to the river’s downstream direction. By measuring it, researchers can tell whether a river is developing more vertically or horizontally, and how sharp its bends are, either of which could be influenced by plants.
The researchers compared migration angles across each river system to determine how river bends varied between vegetated and unvegetated rivers. They found that vegetated rivers tend to deposit sediments in the river bend, leading to curvier bends that develop horizontally and widen over time. In contrast, unvegetated rivers deposit sediment downstream, which means the rivers bend less and have greater variability in bend width.
The question remained whether plants were the primary cause of these differences or whether other factors were at play. To resolve this, the researchers investigated 3 additional factors. The first was the natural fluctuations in water flow across a river system, called its flow variability. They found that during storms, flow variability caused river bends to move downstream in unvegetated rivers, but not in vegetated rivers. This result suggested that flow variability alone didn’t drive downstream migration, although it can directly impact vegetation.
The second variable the researchers analyzed was the amount of sediment a river can carry, or its sediment flux. They found that rivers carrying more sediment can erode more banks, also shifting river bends. However, rivers with more sediment but the same level of plant coverage had statistically similar bend angles. Thus, the researchers concluded that sediment flux alone can’t drive bend development, and that the changes were instead dependent on vegetation cover.
The third variable they analyzed was riverbank strength. The researchers observed rivers with strong banks, made of rock or compacted sediment, and weak banks, made of loose sediment. They observed no difference in river bends with the same vegetation cover but different bank strengths. The researchers concluded that bank strength is also not the primary driver of bend migration in vegetated or unvegetated rivers.
Of the 4 variables the researchers examined – flow variability, sediment flux, bank strength, and vegetation cover – vegetation cover consistently had the greatest impact on the appearance of meandering rivers. They concluded that meandering rivers could have existed before plants, but would have looked different. Like modern unvegetated rivers, ancient meandering rivers likely had lower-angle bends. As plants evolved and grew on river banks, the bends would have developed differently, becoming curvier like modern vegetated rivers. They suggested that understanding this process provides insight into life on Earth before plants evolved 450 million years ago.
