The shape of a river may seem trivial, but it has far-reaching effects on the life in and around it. The bends in a sinuous channel, for example, can alter the water’s temperature or chemistry, making it different from sections that run in a straight line and creating new microenvironments that plants and animals need to adapt to, Davies says.
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Even the earliest plants, which resembled mosses, could have begun to alter how sediments accumulate on riverbanks, says Kevin Boyce, a paleontologist at Stanford University who co-wrote about the evolution of plants in the 2017 Annual Review of Earth and Planetary Sciences. “Those weren’t big trees,” Boyce says, “but they still would have influenced the movements of water” by slowing its flow. As plants evolved to become tree-sized by about 390 million years ago, they gained the power to slow wind. Fine particles caught up in winds would drop to the ground when gusts died in the branches, leaving more sediment caught among trunks and stems.
This posed new challenges to animals like early millipedes and wormlike creatures. “Mud is providing a totally different medium for things to live in,” says Anthony Shillito, a geologist at the University of Oxford.
To get through mud, an animal such as a worm creates cracks to shuffle through by contracting its body, extending it, squeezing water out of the way, and moving forward. Shillito notes that this is mechanically different from traveling through sand, which requires an animal to excavate material out of the way. So early land worms and insects would have had to evolve body parts equipped to deal with muckier movements.
And those movements, in turn, could have helped shape the mud itself, says Lidya Tarhan, a paleobiologist at Yale University. “The act of digging and excavating those burrows and keeping them clear can move around sediments and change the distribution of sediments, and also affect the chemistry,” she says. For example, some invertebrates ingest sediments to extract nutrition, and the chemical reactions in their guts can form fine particles that come out in their feces as mud.
But the strongest influence that early burrowing animals likely had on their muddy environments, Tarhan says, would have been loosening up mud and letting it disperse within rivers and across landscapes. With the rise of single-threaded rivers, mud would have had more opportunities to spread onto floodplains. Such plains don’t develop as easily alongside braided rivers, whose banks easily collapse as waters rise, says Chris Paola, a sedimentologist at the University of Minnesota.
Modern rivers that people have deforested illustrate how the absence of vegetation can destabilize riverbanks and make them less cohesive. Along California’s Sacramento River, for example, areas that farmers cleared for cropland are far more susceptible to erosion than areas that remain forested. Conservationists have worked to stabilize that river by planting more than a million seedlings along its banks.