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Silt: Not so Boring After All


by Josh Jack

Silt is a substance that has rarely stirred much enthusiasm in the public. Perhaps you associate it with rivers. Perhaps your interest ends there. A single grain of silt isn’t much to look at (literally, you might not be able to even see it with the naked eye, though a speck might rub the cornea raw if blown there by the wind) but together they are a force to be reckoned with; have in fact built empires. The kingdom of Egypt, which through all its various dynasties reigned for over 4,000 years, was founded on the banks of the Nile, its soils replenished annually with a thick black muck which was associated with Osiris and rebirth because of its capacity to transform rot into abundance. We encounter this pattern the world over: great rivers, fat with rain and swirling with mud birth civilization: the Yangtze, the Mekong, the Yellow, the Ganges, the Indus, the Congo, the Niger, the Zambezi, the Tigris and Euphrates, the Volga, the Danube, the Amazon, the Rio de la Plata, the Rio Grande, the Colorado, and the Mississippi… Water and silt, it would appear, are the mother’s milk of civilization.

But what is silt? Well, technically speaking it’s a fine-grained (between 0.0039 and 0.0625 mm) particulate finer than sand, but coarser than clay. Normally composed of quartz or feldspar, it’s formed by the wearing away of stone. This normally occurs due to the action of water, wind, or ice (in the form of glaciers.) Once free it’s highly mobile, often conveyed by water, being lighter than stone or gravel, and so much swifter when in suspension. If you’ve ever been caught in a dust storm you’ve encountered silt’s windblown incarnation (geologists call it loess.) Loess can travel quite far and accumulate very quickly, forming deposits tens or even hundreds of feet deep in places like the foothills of the Himalayas, or the bluffs near Vicksburg, Mississippi. Recent studies have demonstrated that much of the Amazon’s fecundity arrives from across the Atlantic Ocean in the form of loess scoured from the Sahara desert; nearly 30 million tons fall across the jungle every year like manna from heaven.

However it arrives silt inevitably finds it’s way back into the hydrosphere. While this may be a boon for agriculture, it has not always been convenient for those who would harvest the crops. The Yellow River, for instance, is also known as China’s Sorrow. It’s waters, leaden with the yellow loess from which it gets its English name, will silt up suddenly and without warning, causing the river to change course abruptly and flatten anything in its path. The Yellow River is known to have altered its course 26 times in recorded history, the most sever shift sending its waters into the Yellow Sea over 300 miles from the location of its former mouth. Catastrophic floods have scoured the area for millennia, but the deadliest occurred in 1323 when as many as 7 million people are known to have died.

But silt can do more than build and destroy civilizations—it can change more than the course of rivers and human destiny—it can actually shape the earth and the sea itself. People unaccustomed to the swamps and bayous of Southeast Louisiana might be tempted to think of them as a vast primordial landscape old as time. They couldn’t be more wrong. By the most conservative estimates The entire southern half of the state of Louisiana is les than 7,500 years old. The Mississippi river, with the aid of over 100 million tons of silt a year, have thrust land ever farther into the sea, braiding the muck into bars, and ropes, and islands. This river too is known to change its course. The Delta is the product of at least six distinct ‘delta complexes’ which laid new land in great balloon-shaped lobes before shifting course again. The river settled into its current channel only around three hundred years ago, just as Europeans were in the process of ‘discovering’ the area. For the last fifty years, it has been trying to alter its course again, into the Atchafalaya basin (itself a distributary of the Mississippi which was dredged and widened by people to make it more navigable and, as it turns out, a more appealing route for the river) kept in place only by the current system of levies and the monumental work of the Army Corps of Engineers, whose Old River Control Station shunts fully one third of the Mississippi’s flow into the Atchafalaya. However, in their attempts to defer the high cost that silt will inevitably exact they have spent most of its many blessings. Contained, the Mississippi Delta withers, its lands being washed away more quickly than they can be rebuilt or replaced. The silt-chocked waters of the river seek commerce elsewhere: at the bottom of the Gulf where they might foster benthic marvels of which we could scarcely conceive, or be carried still further by some errant current or geologic upthrust to seed the coats of Africa or Guam

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