silt and flooding
Feb. 18th, 2013 12:02 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
thanateI finished up the Wormwood Forest book for TV-side reading (which was very cool, by the way) and have moved on to the ![[livejournal.com profile]](https://www.dreamwidth.org/img/external/lj-userinfo.gif)
mrissa-recommended The Early Chinese Empires: Qin and Han for something to pair with wii fit free step. (Did I mention I'd pulled out the wii fit as alternate exercise for when it's too cold to walk outside? It works well as a metabolism boost, but is a bit harder on the calf muscles, as stepping on and off the silly platform doesn't do so well with stretching. But I can read at the same time...) Anyway. I've barely started the book, but I got caught by the bit of introduction about the Yellow River.
I had known that the Yellow River has been diked up/in over the centuries to the point where the bottom of the river is actually above the surrounding countryside in places. What I hadn't been aware of is how much this is a problem of the particular river's geology, rather than just several thousand years of the kinds of sedimentation issues I'm familiar with from around here.
For those of you without the context, this is a river in touch with its watershed:
(It's the Laramie, going through the Rawah valley in northern CO) Key features include low banks over which the water can spread out in flood conditions, and a distinct lack of obvious mud ready to be cut away.
The mid-Atlantic region (and most of the US, really) is characterized by waterways disturbed by a couple centuries of plow and grazing based agriculture and a whole lot of construction that replaces former woods, meadows, and marshes first with bare dirt and then with impervious surfaces (pavement, rooftops.) This means that a large percentage of the water going into our rivers is from run-off rather than groundwater, which means it's warmer, moves faster, and carries more soil with it than what the rivers are used to carrying. The temperature is more an issue for aquatic species than river flow in general, but the faster waterflow and larger volume coming out of your neighborhood storm sewer when it rains magnify the erosion and siltation problems. You've probably all seen some stream in a sliver of woods somewhere that flows in a deep channel cut between muddy banks; this is not actually normal to most landscapes without human intervention. And the various aging dams along our east coast rivers compound the problem in ways that probably aren't relevant here.
In any case, according to Lewis this is not what's going on with the Yellow River:
Moving swiftly in narrow channels through the mountains, the Yellow River carries off a great deal of soil. In most rivers in the world, a silt content of 5 percent is considered high, but the Yellow River has been known to carry as much as 46 percent, and one of its tributaries carries 63 percent. This huge concentration of silt makes the water murky and explains the origin of the river's name. For the last 500 miles of its course, there are no major tributaries, so the river slows down and deposits its silt as sediment.
Over time, as the bottom of the channel gradually rose, the river overflowed its banks. Dikes were built ever higher to prevent flooding, and in some places the river started to flow above the surrounding countryside. Today, in a stretch of about 1,100 miles, the Yellow River moves along 11 yards above the plain. But dikes do not control silting, and floods continue to occur on an even larger scale. On more than 1,500 occasions during the history of imperial China the Yellow River burst its dikes, destroying farmland, killing villagers, and earning its description as "China's Sorrow."
(Also preserving ancient bowls of noodles.)
It makes me wonder about what's going on in those mountain tributaries, and also what the silt percentages are on our own Big Muddy and how much of that is from the central plains geology vs recent human intervention. (See also John McPhee's essay from The Control of Nature about re-routing the Mississippi so that it doesn't find itself a new main channel through its delta that messes with the commercial shipping routes.) The internet is thus far less than helpful to my cause, though I actually think I might know someone to ask, when I get to a point where calling someone up and saying, hey, would you like to meet up for lunch sometime? doesn't seem like a particularly silly idea.
(ETA: also somewhat related: an interesting article on dredge and urban land sculpting)
mrissa-recommended The Early Chinese Empires: Qin and Han for something to pair with wii fit free step. (Did I mention I'd pulled out the wii fit as alternate exercise for when it's too cold to walk outside? It works well as a metabolism boost, but is a bit harder on the calf muscles, as stepping on and off the silly platform doesn't do so well with stretching. But I can read at the same time...) Anyway. I've barely started the book, but I got caught by the bit of introduction about the Yellow River. I had known that the Yellow River has been diked up/in over the centuries to the point where the bottom of the river is actually above the surrounding countryside in places. What I hadn't been aware of is how much this is a problem of the particular river's geology, rather than just several thousand years of the kinds of sedimentation issues I'm familiar with from around here.
For those of you without the context, this is a river in touch with its watershed:
(It's the Laramie, going through the Rawah valley in northern CO) Key features include low banks over which the water can spread out in flood conditions, and a distinct lack of obvious mud ready to be cut away.
The mid-Atlantic region (and most of the US, really) is characterized by waterways disturbed by a couple centuries of plow and grazing based agriculture and a whole lot of construction that replaces former woods, meadows, and marshes first with bare dirt and then with impervious surfaces (pavement, rooftops.) This means that a large percentage of the water going into our rivers is from run-off rather than groundwater, which means it's warmer, moves faster, and carries more soil with it than what the rivers are used to carrying. The temperature is more an issue for aquatic species than river flow in general, but the faster waterflow and larger volume coming out of your neighborhood storm sewer when it rains magnify the erosion and siltation problems. You've probably all seen some stream in a sliver of woods somewhere that flows in a deep channel cut between muddy banks; this is not actually normal to most landscapes without human intervention. And the various aging dams along our east coast rivers compound the problem in ways that probably aren't relevant here.
In any case, according to Lewis this is not what's going on with the Yellow River:
Moving swiftly in narrow channels through the mountains, the Yellow River carries off a great deal of soil. In most rivers in the world, a silt content of 5 percent is considered high, but the Yellow River has been known to carry as much as 46 percent, and one of its tributaries carries 63 percent. This huge concentration of silt makes the water murky and explains the origin of the river's name. For the last 500 miles of its course, there are no major tributaries, so the river slows down and deposits its silt as sediment.
Over time, as the bottom of the channel gradually rose, the river overflowed its banks. Dikes were built ever higher to prevent flooding, and in some places the river started to flow above the surrounding countryside. Today, in a stretch of about 1,100 miles, the Yellow River moves along 11 yards above the plain. But dikes do not control silting, and floods continue to occur on an even larger scale. On more than 1,500 occasions during the history of imperial China the Yellow River burst its dikes, destroying farmland, killing villagers, and earning its description as "China's Sorrow."
(Also preserving ancient bowls of noodles.)
It makes me wonder about what's going on in those mountain tributaries, and also what the silt percentages are on our own Big Muddy and how much of that is from the central plains geology vs recent human intervention. (See also John McPhee's essay from The Control of Nature about re-routing the Mississippi so that it doesn't find itself a new main channel through its delta that messes with the commercial shipping routes.) The internet is thus far less than helpful to my cause, though I actually think I might know someone to ask, when I get to a point where calling someone up and saying, hey, would you like to meet up for lunch sometime? doesn't seem like a particularly silly idea.
(ETA: also somewhat related: an interesting article on dredge and urban land sculpting)