You may know about this than I do, but the current would bring the silt
down to the sea over time. If it just remained at the bottom of the
river, there would be no problem.
On Wed, Jun 27, 2001 at 03:03:42PM -0700, Tim Bousquet wrote:
> Yes, but doesn't the high-silt content of the river
> date back several thousand years at least? Isn't it
> pretty much the prehistoric, natural state of the
> river?
>
> --- Michael Perelman <[EMAIL PROTECTED]>
> wrote:
> > Reforestation can actually reduce the size of
> > deserts over time.
> >
> > On Wed, Jun 27, 2001 at 02:37:47PM -0700, Tim
> > Bousquet wrote:
> > > How do you "take care of" the Gobi Desert?
> > >
> > > tim
> > > --- Michael Perelman <[EMAIL PROTECTED]>
> > > wrote:
> > > > You can't tame rivers, but you can take care of
> > the
> > > > land on the hillsides
> > > > to reduce siltation.
> > > >
> > > > Ricardo Duchesne wrote:
> > > >
> > > > > I have come to the conclusion that China's
> > > > hydraulic lock-in and
> > > > > long term patter of development cannot be
> > fully
> > > > grasped without a
> > > > > clear appreciation of the ecological dynamic
> > of
> > > > the Yellow River.
> > > > > This, the most unsubordinate, intractable,
> > > > turbulent river of the
> > > > > world, has long driven a hard bargain. In
> > exchange
> > > > for its heavily
> > > > > sedimented water, the fruit that nourished
> > the
> > > > Shang, the Zhou,
> > > > > the Chhin, the Han, the Sui, and the Tang
> > > > civilizations (to 907AD),
> > > > > it killed more people than any other river in
> > the
> > > > world.
> > > > >
> > > > > Here are some facts on file:
> > > > >
> > > > > -- The Hwang Ho, with a lenth of 2,900 miles,
> > is
> > > > China's second
> > > > > largest river, and the 10th longest in the
> > world.
> > > > >
> > > > > --Carrying up to 40 percent sediment by weight
> > (in
> > > > some stretches
> > > > > as much as 60 percent), it is the most
> > silt-laden
> > > > river of the world.
> > > > > Sediment over 10 percent by weight is very
> > rare
> > > > among the large
> > > > > rivers of the world; even 2 to 3 percent is
> > high
> > > > (Cressy, 1955).
> > > > > Much of the silt is loess, a fine wind-blown
> > soil
> > > > which the river
> > > > > picks up in its upper course as it flows
> > through
> > > > the "yellow earth"
> > > > > region. "Much of this silt is then dropped in
> > its
> > > > more sluggish lower
> > > > > reaches, building up the height of the river
> > bed,
> > > > and making its
> > > > > course unstable" (Blunden and Elvin, 17)
> > > > >
> > > > > --While the Yangzi River discharges a greater
> > > > volume of water, the
> > > > > Hwang Ho is much more unstable in its flow; at
> > low
> > > > water, the flow
> > > > > may drop to 5000 cubic feet per second; in
> > flood
> > > > it can reach
> > > > > 1,000,000 (Cressy). But the most challenging
> > > > engineering aspect
> > > > > is control of the exceptionally high sediment.
> > As
> > > > the river passes
> > > > > through the loess lands and erodes the loess,
> > it
> > > > becomes a river of
> > > > > yellow mud which is then deposited across the
> > > > North China Plain.
> > > > >
> > > > > -- To deal with this shifting, sediment-loaded
> > > > river, dykes were built,
> > > > > to keep the water stable, but as a result of
> > dike
> > > > building, the
> > > > > surplus sediment which nature would have
> > otherwise
> > > > spread far
> > > > > and wide has been confined between artificial
> > > > barriers. Thus the
> > > > > bed of the river has been continually raised
> > > > requiring dikes to be
> > > > > built higher and higher. This dyking has gone
> > on
> > > > indefinetely.
> > > > > Millions now live below the level of the diked
> > > > floor water. Areas
> > > > > miles from the river have elevations many feet
> > > > below the bottom of
> > > > > the river.
> > > > >
> > > > > --But this river refuses to be tamed. Not only
> > has
> > > > the river's levees
> > > > > been breached thousands of times, its lower
> > course
> > > > has changed
> > > > > 26 times in China's history. A highly
> > devastating
> > > > change of course
> > > > > occurred in 1194 AD when flood water rushed
> > onto
> > > > the Huai River
> > > > > basin taking over this river's drainage system
> > for
> > > > the next 700 years
> > > > > (Leung, 1996). Siltation at the mouth of the
> > River
> > > > has extended the
> > > > > length of the river by about 35 miles betweern
> > > > 1975 and 1991.
> > > > >
> > > > > -- By the 1950s the northwest province of
> > Shensi
> > > > had 13 modern
> > > > > canal systems, with a total length of 600
> > miles.
> > > > Currently the
> > > > > Chinese are constructing a massive new dam
> > called
> > > > Multipurpose
> > > > > Dam Project with 10 intake towers, nine flood
> > and
> > > > sediment
> > > > > tunnels, six power tunnels and an underground
> > > > powerhouse.
> > > > >
> > > > > --The floods of Hwang Ho are the most
> > destructive,
> > > > since they
> > > > > persist for long periods and spread over the
> > > > countryside in every
> > > > > direction (unlike the Mississipi where the
> > flooded
> > > > areas are usually
> > > > > a ribbon between the river and its bluff).
> > When
> > > > the flood ends, a
> > > > > veneer of sand and mud covers everything
> > except
> > > > for the few tree
> > > > > tops which had remained above water. While the
> > > > Egyptians referred
> > > > > to the annual flooding of the Nile as the
> > "Gift of
> > > > the Nile", the
> > > > > Chinese have nicknamed their unruly Hwang Ho
> > > > "China's Sorrow".
> > > > >
> > > > > --When the river's current slows, and the
> > river
> > > > loses its carrying
> > > > > power, excessive sedimentation takes place
> > within
> > > > a few days. The
> > > > > bed of the river is thus raised. When the next
> > > > flood arrives, dikes
> > > > > are overtopped before there is a chance for
> > the
> > > > increased
> > > > > movement of the water to excavate previous
> > > > accumulations.
> > > > >
> > > > > -- No amount of dyking can entirely eliminate
> > the
> > > > wide variations in
> > > > > its flow, which to an extent seen nowhere else
> > in
> > > > the world it is
> > > > > also a flow of mud. Each year 1,890, 000, 000
> > > > metric tons of silt
> > > > > are brought to the head of the delta plain.
> > "The
> > > > control of the
> > > > > Hwang Ho is surely one of the most baffling
> > > > hydrologic problems on
> > > > > earth; were the river in the United States it
> > > > would tax all the
> > > > > country's financial resources and engineering
> >
> === message truncated ===
>
>
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--
Michael Perelman
Economics Department
California State University
Chico, CA 95929
Tel. 530-898-5321
E-Mail [EMAIL PROTECTED]
