阅读理解
Rivers are
earthly arteries(要道) for the
nutrients, deposits and freshwater that sustain healthy, diverse ecosystems.
Their influence extends in multiple dimensions—not only along their length but
belowground to aquifers(蓄水层) 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 draw off water for
drinking and irrigation, humans have increasingly constrained and broken
these crucial water ways. “We try to control rivers as much as possible,” says
Gunther Grill, a hydrologist at McGill University.
In new
research published in May in Nature, Grill and his colleagues analyzed the
barriers to 12 million total kilometers of rivers around the world. The team
developed an index(指数) 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 dimension. Rivers whose indexes meet a certain threshold(临界值) for being largely able to follow their natural
patterns were considered freeflowing.
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 not
blocked along their entire lengths. Most of them are in areas with a
minimal human presence, including the Amazon and Congo basins and the Arctic.
On the contrary, 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(河口) and deltas
(such as the Mississippi Delta) of key nutrients. The world's estimated 2.8
million dams are the main cause, controlling water flow and trapping deposits.
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.”