FLOODING
AND ITS IMPACT
Floods can occur in rivers when the flow rate exceeds the capacity of
the river channel, particularly at bends or meanders in the waterway.
Floods often cause damage to homes and businesses if they are in the
natural flood plains of rivers. While riverine flood damage can be
eliminated by moving away from rivers and other bodies of water, people
have traditionally lived and worked by rivers because the land is
usually flat and fertile and because rivers provide easy travel and access
to commerce and industry.
|
A
|
Fire and
flood are two of humanity’s worst nightmares. People have,therefore,always
sought to control them. Forest fires are snuffed out quickly. The flow of
rivers is regulated by weirs and dams. At least, that is how it used to be.
But foresters have learned that forests need fires to clear out the brash
and even to get seeds to germinate. And a similar revelation is now –
dawning on hydrologists. Rivers – and the ecosystems they support – need
floods. That is why a man-made torrent has been surging down the Grand
Canyon. By Thursday March 6th it was running at full throttle, which was
expected to be sustained for 60 hours.
|
B
|
Floods once
raged through the canyon every year. Spring Snow from as far away as
Wyoming would melt and swell the Colorado river to a flow that averaged
around 1,500 cubic metres (50,000 cubic feet) a second. Every eight years
or so, that figure rose to almost 3,000 cubic metres. These floods infused
the river with sediment, carved its beaches and built its sandbars.
|
C
|
However, in
the four decades since the building of the Glen Canyon dam, just upstream
of the Grand Canyon, the only sediment that it has collected has come from
tiny, undammed tributaries. Even that has not been much use as those
tributaries are not powerful enough to distribute the sediment in an
ecologically valuable way.
|
D
|
This lack of
flooding has harmed local wildlife. The humpback chub,for example, thrived
in the rust-redwaters of the Colorado. Recently, though, its population has
crashed. At first sight, it looked as if the reason was that the chub were
being eaten by trout introduced for sport fishing in the mid-20th century.
But trout and chub co-existed until the Glen Canyon dam was built, so
something else is going on. Steve Gloss, of the United States’ Geological
Survey (USGS), reckons that the chub’s decline is the result of their
losing their most valuable natural defense, the Colorado’s rusty sediment.
The chub were well adapted to the poor visibility created by the thick, red
water which gave the river its name, and depended on it to hide from predators.
Without the cloudy water the chub became vulnerable.
|
E
|
And the chub
are not alone. In the years since the Glen Canyon dam was built, several
species have vanished altogether.
These include the Colorado pike-minnow, the razorback sucker and the
round-tail chub. Meanwhile, aliens including fathead minnows, channel
catfish and common carp, which would have been hard, put to survive in the
savage waters of the undammed canyon, have move din.
|
F
|
So flooding
is the obvious answer. Unfortunately, it is easier said than done. Floods
were sent down the Grand Canyon in 1996 and 2004 and the results were
mixed. In 1996 the flood was allowed to go on too long. To start with,all
seemed well. The floodwaters built up sandbanks and infused the river with
sediment. Eventually, however, the continued flow washed most of the
sediment out of the canyon. This problem was avoided in 2004, but unfortunately,
on that occasion, the volume of sand available behind the dam was too low
to rebuild the sandbanks. This time, the USGS is convinced that things will
be better. The amount of sediment available is three times greater than it
was in 2004. So if a flood is going to do some good, this is the time to
unleash one.
|
G
|
Even so, it
may turn out to be an empty gesture. At less than 1,200 cubic metres a
second, this flood is smaller than even an average spring flood, let alone
one of the mightier deluges of the past. Those glorious inundations moved
massive quantities of sediment through the Grand Canyon,wiping the slate dirty,
and making a muddy mess of silt and muck that would make modern river
rafters cringe.
|
Questions 1-7
Do the following statements
agree with the information given in Reading Passage 1?
In boxes 1-7 on your
answer sheet, write
TRUE
|
if the
statement is true
|
FALSE
|
if the
statement is false
|
NOT GIVEN
|
if the information is not
given in the passage
|
1
|
Damage caused
by fire is worse than that caused by flood.
|
2
|
The flood
peaks at almost 1500 cubic meters every eight years.
|
3
|
Contribution of sediments
delivered by tributaries has little impact.
|
4
|
Decreasing
number of chubs is always caused by introducing of trout since mid 20th
century.
|
5
|
It seemed that the artificial
flood in 1996 had achieved success partly at the very beginning.
|
6
|
In fact, the
yield of artificial flood water is smaller than an average natural flood at
present.
|
7
|
Mighty floods drove fast
moving flows with clean and high quality water.
|
Questions 8-13
Complete the summary below.
Choose NO MORE THAN TWO WORDS
from the passage for each answer.
Write your answers in boxes 8-13
on your answer sheet.
The
Eco-Impact of the Canyon Dam
Floods are people’s nightmare. In the past, canyon was raged by flood
every year. The snow from far Wyoming would melt in the season of 8___________________
and caused a flood flow peak in Colorado river. In the four decades after
people built the Glen Canyon dam, it only could gather 9___________________
together from tiny, undammed tributaries.
Humpback chub
population on reduced, why?
Then, several species disappeared including Colorado pike-minnow, 10___________________
and the round-tail chub. Meanwhile,
some moved in such as fathead minnows, channel catfish and 11___________________.
The non-stopped flow leaded to the washing away of the sediment out of the
canyon, which poses great threat to the chubs because it has poor 12___________________
away from predators. In addition, the volume of 13___________________ available behind the dam was too low to
rebuild the bars and flooding became more serious.
|
|
No comments:
Post a Comment
thank you for visiting my blog and for your nice comments