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Rivers as we know them

Plants also appear to have had a hand in shaping the face of the planet. In the Cambrian Period, more than 500 million years ago, rivers were broad and shallow, and laid down wide, flat sheets. Traces of their banks are elusive in the geologic record. Some have estimated they had width-to-depth ratios on the order of 1,000 to 1, according to Martin Gibling, a professor of earth sciences at Dalhousie University.

The evolution of land plants, along with some help from mud, ultimately gave rivers the sinuous, narrow channels, islands, muddy floodplains and the species-rich corridors associated with modern rivers, at least those left in their natural state, according to Gibling and colleague Neil Davies of the University of Ghent in Belgium. Most modern rivers do not follow their natural courses as a result of human modifications. [Humans Originated Near Rivers]

As the plants caused changes in the riversand the areas around them — by holding banks in place, dropping in woody debris or contributing to soil formation — they also opened up new opportunities for the plants themselves and for animals, like fish.

The first simple plants appear to have arrived on land by around 470 million years ago. Mud— which is more cohesive than sand — showed up around this time, possibly a little earlier. It gave riverbanks more stability, allowing channels to deepen and eventually follow meandering paths. While mud got this process started, it was plants that got it going, according to Gibling.

Plants contribute to the production of mud by breaking down rock, both with acids and, a little later, with their roots, which further stabilized the river banks.

The first signs of meandering rivers emerge around 416 million years ago. As the rivers changed, this created new opportunities for plants themselves, and for animals, like insects and fish.

Around 320 million years ago, sets of narrow channels with rigid banks, appeared.

"Something happens, and we think this is the conifers," Gibling said, referring to cone-bearing trees. These have deep root systems and they appear in the fossil record at about this time.

Woody debris, which had been showing up long before the conifers arrived, also has important effects on rivers. Modern rivers with logs and snags are narrower, slower-moving, and have more stable banks. (European colonists, seeking to make rivers more navigable, removed logs and debris for the opposite result.) The wood also provides important habitat for freshwater fish.

As trees became more abundant, islands, held in place by their roots, began to appear in rivers more than 300 million years ago.

"Organic matter is often not well-preserved into the geological record. It decays away rapidly – even large trees and logs. So, geologists can easily assume that no plants were present," Gibling told LiveScience in an email, noting there is now enough evidence that plants were widespread by the time modern rivers arose. "We need to think more about how this would have affected landscapes."

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