Rivers are terrestrial arteries for the nutrients, sediment and freshwater that sustain healthy, diverse ecosystems. Their influence extends in multiple dimensions—not only along their length but belowground to aquifers and periodically into nearby floodplains.
They also provide vital services for people by fertilizing agricultural land and feeding key fisheries and by acting as transportation corridors. But in efforts to ease ship passage, protect communities from flooding, and siphon off water for drinking and irrigation, humans have increasingly constrained and fractured these crucial waterways. “We try to tame rivers as much as possible,” says Günther Grill, a hydrologist at McGill University.
In new research published in May in Nature, Grill and his colleagues analyzed the impediments to 12 million total kilometers of rivers around the world. The team developed an index that evaluates six aspects of connectivity—from physical fragmentation (by dams, for example) to flow regulation (by dams or levees) to water consumption—along a river's various dimensions. Rivers whose indices meet a certain threshold for being largely able to follow their natural patterns were considered free-flowing.
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Credit: Melissa Thomas Baum, Buckyball Design; Source: “Mapping the World’s Free-Flowing Rivers,” by G. Grill et al., in Nature, Vol. 569; May 9, 2019
The researchers found that among rivers longer than 1,000 kilometers (which tend to be some of those most important to human activities), only 37 percent are unimpeded along their entire lengths (graphic). Most of these big unhindered rivers are in areas with a minimal human presence, including the Amazon and Congo basins and the Arctic. Conversely, most rivers shorter than 100 kilometers appeared to flow freely—but the data on them are less comprehensive, and some barriers might have been missed. Only 23 percent of the subset of the longest rivers that connect to the ocean are uninterrupted. For the rest, human infrastructure is starving estuaries and deltas (such as the Mississippi Delta) of key nutrients. The world's estimated 2.8 million dams are the main culprit, controlling water flow and trapping sediment.
The new research could be used to better understand how proposed dams, levees and other such projects might impact river connectivity, as well as where to remove these fixtures to best restore natural flow. It could also help inform our approach to rivers as the climate changes, says Anne Jefferson, a hydrologist at Kent State University, who was not involved in the work. Existing infrastructure, she says, “has essentially been built to a past climate that we are not in anymore and are increasingly moving away from.”