Reading: Matching Information to Paragraphs: Question 9

A A drainage basin is described as endorheic, or closed, when the rivers and streams within it never reach the ocean; instead of feeding into the sea as basins draining outward, described as exorheic, typically do, the water within an endorheic system terminates internally, in a lake, marsh or low-lying sink, or is lost entirely to evaporation and seepage into the ground.

B Such closed systems most commonly develop in arid interior regions that are either encircled by mountain ranges high enough to block any river from reaching the coast, or that occupy a low-lying tectonic depression where the surrounding terrain slopes inward rather than toward any ocean, so that whatever rain does fall has nowhere to drain but into the basin's own lowest point.

C Because the only way water leaves an endorheic basin is through evaporation, any minerals and salts carried in by inflowing rivers are left behind rather than being carried out to sea, and over long periods this steady concentration explains why so many of the terminal lakes at the bottom of these basins are markedly saline, sometimes far saltier than the ocean itself.

D Systems of this kind are far from rare: substantial endorheic drainage covers large portions of the interior of Asia, extensive high-altitude sections of the South American Andes, and much of the arid interior of the Australian continent, together accounting for a notable share of the planet's total land area.

E Diverting river water for irrigation within a closed basin carries a particular long-term risk, because none of that water is ever replenished from the sea: as irrigation water evaporates from cultivated fields, the dissolved salts it carried are left behind in the topsoil, and this gradual salinisation can steadily reduce the fertility of farmland that depended on the basin's rivers.

F The ecosystems that develop around the salt lakes of a closed basin tend to be unusually sensitive to any change in the volume of water flowing in, since there is no connection to the open ocean that could dilute a sudden rise in salinity or flush out an accumulation of pollutants, leaving species that are specially adapted to a narrow range of conditions with nowhere to escape a shift in the water's chemistry.

G Because sediment that settles at the bottom of an endorheic basin is never swept away to the sea, the undisturbed layers that accumulate over thousands of years can preserve an exceptionally continuous record of past conditions, and researchers studying historical climate patterns frequently drill into these deposits to read the sequence of layers much as they would examine tree rings.

Which paragraph contains the following information? Write the correct letter, A–G, for each statement. NB You may use any letter more than once.

1. An explanation of why particular kinds of lake tend to become highly saline over time.
2. A description of the geographic conditions typically responsible for the formation of this kind of drainage system.
3. A claim that this type of basin makes an especially reliable long-term record for studying historical climate.
4. A warning that agricultural irrigation can cause gradual land degradation within this type of system.
5. A statement defining how this type of drainage system differs from a more familiar type that flows to the sea.
6. A reference to several distinct regions of the world, on different continents, that contain large systems of this kind.
7. An explanation of why the ecosystems within a closed basin are unusually vulnerable to changes in water inflow.