WE THINK THAT IT IS THE BIG OR THE LITTLE THINGS that make the difference in life: the biggest dam ever, the smallest hole. But the persistent middle term—the stuff that just keeps coming—that is what really changes the landscape.
Clay settles, sand hops, silt slides. Rubbed between two fingers, dry silt feels lubricious. It is the smoothness of a soil. Silt is the middle term between big sand and submicroscopic clay; it mediates between chemistry and structure. It is the soil when it daydreams, pushed this way by the water or that way by the wind, present but not decisive, flighty but not unhinged.
With the soil, as elsewhere, we usually forget the middle term, focusing instead on the extremes. But it is silt that most often travels, and so it is silt that is responsible for the best and worst that soil can do.
If you drop a particle of coarse sand in water, it will fall about four inches in one second. A particle of very fine clay, on the other hand, will take about 860 years to fall the same four inches. Silt will fall the same distance in five minutes.
Silt flew from the trailing edge of retreating glaciers to make the fine black soils of the Midwest. It slides along the streams of the world, leaving deposits that make whole regions fertile. But silt, too, led the other particles north to Canada, when the dust storms of the Great Depression came.
Other conditions being equal, soils rich in silt are the most erodible. Raindrops striking the soil surface and running in the plow furrows dislodge silt particles from their dry resting place, knocking them into the pores through which the soil receives the water. The much smaller clays then pile up around these partial blockages, and soon you have a thin crust on the soil surface that prevents water and air from getting in and germinating seeds from getting out.
It is like what happens when you are rinsing off the dinner dishes and the water runs out through one of those perforated catchers that keep the drain from clogging. The spaghetti sauce’s smaller particles pass, until a couple of hunks of onion settle in the catcher; after that, more and more particles are impeded and a cap develops. You curse and get your sleeves wet remedying the situation.
Much of the disastrous erosion of the American Dustbowl began when rain struck unprotected soil, rearranging the silt and clay and starting sheets and gullies of water running across the soil surface. The winter of 1932 was a wet one in Mississippi’s Yazoo River basin. Twenty-seven inches of rain fell, and the flooding was disastrous. Sixty-two percent of the runoff came from bare fields, carrying with it thirty-four tons of soil per acre into the stream. In a nearby oak forest, a little more than an eighth of an inch ran off and only seventy-five pounds of soil per acre were lost. Tree roots are not the reason that the soil is held in place, so much as the tree and plant canopy, including the litter layer with which it covers the mineral soil, is a shield that preserves structure. In Cooper Basin, Tennessee, where smelter fumes had destroyed a hardwood forest, leaving bare soil, the maximum runoff increased from thirty cubic feet per second to 1,263 cubic feet per second. Fires also increase runoff, not only by destroying ground cover, but also because chemical changes in the surface give the ground a negative charge, causing it to repel water molecules.
But this is not silt’s fault. When it is functioning as it ought, it dreams in the rivers of the world, forming the fertile deposits of their floodplains and their deltas. Lifted into suspension in the faster upland waters, it remains there, until in the lowlands, like a glider coming in for a landing, it bumps to a stop. In the dozen major rivers of the world, it accounts for more than four and half billion tons of floating matter each year. To equal the silt output of the Mississippi River alone, you would have to run almost 25,000 railroad cars full of silt to the Gulf of Mexico. Daily.
Until recently, silt’s annual deposition along the banks of the Nile River accounted for the longest-lived prosperous agriculture in the Western world. Then, in 1980, the Aswan High Dam was opened. Like any dam, it arrests silt. Velocity and gradient drop to zero at the head of the dam’s still-water zone and the silt settles, falls out of suspension, begins to back up. It is not a good end to silt’s dream.
In fact, the invention of dams has changed the word “silt” from a noun to a verb. Now a dam is said to “silt up” or to suffer “siltation.” A little dam like the Mono Reservoir Dam, built in 1935 in California, can silt up quickly. The steep bare landscape around it had been denuded by a forest fire. In two years, the silt reached the top lip of the dam. It will take Egypt’s Lake Nasser very much longer than that—perhaps centuries—to fill, but someday it will.
In the meantime, the water runs clear and pure from the Aswan High Dam, after spinning through the turbines where it generates ten billion kilowatts of power per annum. The loss of the silt downstream, however, is devastating. A million acres of farmland now must be fertilized with industrially produced amendments. Furthermore, the clean water is no longer saturated with sediments, so it gathers a fresh load of mineral matter downstream, increasing erosion below the dam and requiring the total reconstruction of its infrastructure, including four smaller dams and over five hundred bridges. Because sediments no longer reach the sea, the delta is eroding at a rate unprecedented in four thousand years, and the catch of fish, dependent on the nutrient-rich sediment that fed invertebrates, has declined by more than a fifth.
But silt will have its way. Eventually, it will dream itself even to the summit of the Aswan High Dam, and the great project will become a spectacular waterfall, a natural wonder of a different order.