Plants slow river migration

The appearance of land plants in Earth’s fossil record 440 million years ago coincides with another dramatic change in our planet’s geological history. Whereas once broad, sandy floodplains were etched by shallow braided streams, the landscape became dominated by muddy floodplains carved by deep river channels. The spread of terrestrial vegetation stabilizes riverbanks against the high stresses exerted by water flow. The confined flow is deflected along the channel banks, where it simultaneously erodes the outer banks and deposits sediment on the inner banks; the activity leads to meandering rivers whose positions change over time. But on modern Earth and Mars, deep meandering channels also found in barren landscapes suggest that vegetation alone does not control a planet’s past or present hydrological activity.

Alessandro Ielpi of Laurentian University in Sudbury, Ontario, Canada, and Mathieu Lapôtre at Stanford University have now quantified how vegetation influences river mobility and proposed how river movement may have influenced ancient Earth’s climate. Using time-lapse aerial imagery corroborated by fieldwork, the researchers calculated changes in lateral migration rates of 41 meandering rivers around the world for time spans of up to 50 years. Ielpi and Lapôtre focused on rivers in the arid interiors of continents. When normalized for channel width, streams with no vegetation migrated an order of magnitude faster than ones with vegetation. Plants not only provide mechanical strength to channel banks through their roots, but they increase the production of clays in a floodplain. The cohesive, stiff clays further reduce bank erodibility.

The findings suggest that slower migration caused by land plants enhances soil carbon storage: The more vegetation there is along a river’s edge, the longer the river takes to shift position and shape. As a result, vegetated rivers promote carbon storage—and eventual capture—in the soil. In contrast, unvegetated rivers expose buried carbon more frequently and release carbon dioxide into the atmosphere. Similar analyses could help to extrapolate the migration rates of ancient, large, unvegetated channels on Earth and Mars and thus provide insights into past landscapes and environments. (A. Ielpi, M. G. A. Lapôtre, Nat. Geosci., 2019, doi:10.1038/s41561-019-0491-7 ; American Geophysical Union Fall Meeting , 12 December 2019.)