A “Resource Out of Place”
The costs of sedimentation pile up
This the third part of the six-article series that appeared in Illinois Issues magazine between September 1981 and February 1982 and was later published as 48-page booklet titled Breadbasket or Dustbowl. (See Publications above.) The topic this month was the effects off-farm of topsoil washed into drainage ditches and reservoirs. I’d witnessed the pernicious effects of this soil movement in the backwaters of Lake Springfield, which I loved to explore in a kayak until they were buried in soil from the adjacent farm fields.
The series was another of that magazine’s examinations of pressing environmental issues, in this case funded in large part by The Joyce Foundation, with the assistance of the Illinois Department of Agriculture's Division of Natural Resources. The loss of topsoil, from erosion and indeed of prime farmland itself through development had set off alarms in Illinois of the period. The panic proved, shall we say, premature. Subsequent changes in the ways that Illinois farmers tilled their land caused huge (and largely unexpected) declines in topsoil loss.
Originally titled, "Illinois' soil: a "resource out of place." For other articles in the series, see "Breadbasket or Dustbowl" on the Farms & farming page.
Illinois Issues introduction: Erosion 'bleeds' Illinois of 180 million tons of topsoil per year. While some of this soil ends up in the Gulf of Mexico, much of it stays in Illinois—clogging our streams and rivers, filling our lakes and reservoirs and driving away our fish and wildlife. The following article (third in a series funded by The Joyce Foundation) explores the mechanics of soil erosion and the attendant problem of sedimentation.
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Looked at with the educated eye of a geologist, the pristine Illinois prairie so often compared to Eden by visiting Europeans would have been seen as an already degraded landscape. The rolling topography of the state is not a gift of the glaciers, as popularly supposed. It is a more recent invention, the result of water and wind chewing away at once-level plains of loess—the pulverized rock that was the ice sheets' real legacy. Nature sculpted Illinois using erosion as a chisel; idyllic river valleys are in fact the scars of past geologic violence.
Erosion is a natural process and, when acting in an ordered ecology such as ancient Illinois’s, a slow and fairly benign one. But when it acts in an ecology disordered by humans, erosion becomes a wholly unnatural destroyer. Generally speaking, erosion is the transfer of soil and rock from one place to another, usually to a place where it isn't wanted. Water, wind, and ice are the busiest movers, although horses' hooves, steam shovels and off-road vehicles can each accomplish much the same end.
In temperate climates such as Illinois’s, where rainfall averages up to 48 inches a year, water is the principal agent of erosion. (Erosion rates in Illinois tend to vary with annual rainfall, with both being highest in the southern third of the state.) Water erosion begins with the single raindrop. Seemingly harmless, raindrops explode on bare soil like bombs. An inch of water falling over a single acre weighs 100 tons; a two-inch rain in an hour packs the energy-equivalent of 250 horsepower per acre—enough to lift the top seven inches of soil (which weighs roughly 150 tons per acre-inch) to a height of three feet, 86 times.
Raindrops thus cause "splash erosion." A raindrop can blast soil particles two feet into the air and drop them as far as five feet away. On a flat field, the net effect of splash erosion is to leave topsoil pretty much where it was originally. On a sloping field, however, a majority of those airborne soil particles will land downslope according to a precise mathematical formula: on a field with a 10 percent slope, for example, 60 percent of such particles will land downslope.
Soil particles dislodged by this barrage can thus be "kicked" downhill, a few feet at a time. But once loosened, soil can also be washed downhill by sheets of water running off the surface. "Sheet erosion" draws its power from gravity; the steeper the grade, the faster the flow. The water sheet is itself erosive to some extent, but mainly it acts as a carrier of soil particles dislodged by the very much more destructive raindrops, which pack up to 100,000 times as much kinetic energy. The pounding of raindrops falling on water sheets adds to the latter's churning action and enhances their ability to move soil.
Gradually, runoff water collects in channels. Its volume and velocity increase, causing "rill erosion." Particles already in the runoff streams act like miniature rasps, ripping soil particles from the walls of channels while the water itself lifts others from the channel bottom or drags them along. Small rills coalesce into larger ones, magnifying their destructive effects, with the largest of them leaving gashes in the earth's hide known as gullies.
The efficiency with which rainfall goes about this excavation work depends partly on the slope of the land, partly on the nature of soils that lay atop it. Mostly it depends on vegetable cover. Soils covered by blankets of trees, fallen leaves or thick grass are largely shielded from the concussive impact of raindrops, and thus are relatively impervious to erosion. Runoff water from such land is typically unclouded by soil particles. Runoff from bare land, on the other hand, is discolored by the soil it carries. It is almost as if the land were bleeding.
The damage done by wind is less extensive but still real. In the spring of 1981, for instance, rainfall amounts were lower and winds higher than usual. The dry weather tempted many farmers to prepare seedbeds early. Cultivation broke up the earthen clumps which act as tiny windbreaks and reduced to powder the protective scab of matted plant matter and soil on the surface. Thus exposed, the soil was easy prey to winds. (Winds gusting up to 40 miles per hour can strip a field of up to five tons of topsoil in a day. The accepted tolerable rate of soil loss—the rate at which nature can replenish lost soil through its own processes of soil formation, known as the "T" level—is no more than five tons per year.) The resulting dust clouds threatened to close some airports in western Illinois because of reduced visibility. The dust posed political risks as well. As one Illinois Farm Bureau official remarked to a reporter, "Lobbyists for environmental concerns in Springfield see [the dust]. Legislators see it as they drive to their homes. . . . and assume farmers are doing a cruddy job"—a not altogether unfair assumption in this case.
Regardless of cause, the extent of what the Illinois Environmental Council has called "a crime against our fertile land" is immense. The evidence is colloquial but persuasive: fence posts installed at the feet of sloping fields 20 years ago now half-buried in mud, ponds that never used to go dry suddenly going dry every summer, down-state children who grow up thinking that brown is water's natural color.
The direct measurement of erosion losses from "nonpoint" sources such as farm fields being virtually impossible, experts use the Universal Soil Loss Equation (USLE) to estimate erosion loss according to the rainfall, soil type, slope and tillage methods practiced on a given piece of land. Using the USLE (verified by field data from test plots), the Illinois Environmental Protection Agency (IEPA) has estimated that the average annual soil loss per acre in Illinois is 11.7 tons, compared to the "T" level of soil loss (at which topsoil is theoretically sustainable forever) of from two to five tons per acre. Statewide, the losses amount to roughly 180 million tons a year. That is enough to resod every square inch of Cook County every spring.
What is the source of this torrent of mud? In a state in which four-fifths of the surface is used for farming, it is no surprise to learn that most of it comes from farms. According to revised figures originally prepared as part of the Illinois Conservation Needs Inventory, by the University of Illinois, in 1977 gully erosion accounted for barely 6 percent of the state's total soil loss. Stream banks, nonfarm rural land, urban areas, and federally owned land such as national forests were the source of only another 8 percent. Sheet and rill erosion from farmland, however, accounted for more than 86 percent of the state's total losses, with most of that amount (76 percent of the total) coming from cropland.
It is something of a surprise, however, to learn that most of the state's lost topsoil comes from its flattest (2-4 percent slope) land. This is not because erosion losses are particularly high on such land, but because there is so much of it in the state. Also, it is well to remember that a lot of erosion doesn't appear in the statistics. Soil loss estimates typically count only the soil that leaves a field; they do not include, to take a common example, topsoil which is washed off the crown of a hill into adjacent low spots in the same field. Nor, since the experts compute only net soil loss, do such estimates reflect the fact that a field may lose topsoil without losing it. There are an estimated 13 million Illinois acres on which soil losses are at or below the five-ton-per-acre "T" level and where, depending on the field, it is possible for nature to keep pace with soil losses.
The cost to the soil
Illinois’s landscape, then is constantly being resculpted under the sharp chisels of its farmers. The modern system of industrialized agriculture produces more food at lower cost with fewer people than ever before, but only if one does not include in the bill the cost to the soil. Along with rising quantities of diesel fuel, weed killers and fertilizers, every bushel of corn harvested in Illinois in recent years has cost farmers a bushel and a half of topsoil. Ecologist David Brower, speaking at Sangamon State University last March, observed, "If the earth were the size of an egg, all the water on earth would be a drop, and the soil would be a spec, invisible to the naked eye." Estimated losses vary from county to county, but from one-third to one-half of Illinois’s share of that vital speck has already vanished.
Where did it go? Some of it stays on the farm, accumulating in low spots of fields or filling in irrigation and drainage ditches, leaving bald spots on high ground caused by the exposure of lighter colored subsoils. Much of it leaves the farm altogether, settling in city storm sewers and roadside ditches or draining into streams. From there it is carried into reservoirs and ponds, even the Gulf of Mexico. Illinois soil constitutes a significant part of the estimated 400 tons of errant earth that float through the mouth of the Mississippi River every minute.
Suspended in water, soil becomes sediment, a "resource out of place," in agronomic parlance. Eventually it settles onto lake beds and stream bottoms. In Illinois the difference between lake waters and lake bottoms is more one of degree than of kind. Mud is the biggest single pollutant of Illinois lakes and streams. Man-made reservoirs are especially vulnerable, since they have no natural outlets and thus act as giant silt traps for eroded soil washed into them from watersheds that may extend upstream for hundreds of square miles. Lake Springfield is typical. Built in 1935 as a municipal water reservoir, the lake has been slowly filling up ever since, at an average rate of 0.3 percent annually (a fairly modest rate by Illinois standards), so that its water storage capacity has shriveled by 13 percent in all. The cause is an estimated 600,000 tons of topsoil washed into the lake each year from its 265-square-mile watershed. Roughly 90 percent of that watershed area is farmed; indeed, the U.S. Soil Conservation Service estimates that those 600,000 tons represent only 24 percent of the soil washed off the watershed. Dredging the lake to recapture some of this lost storage capacity was estimated two years ago to cost up to $300,000 a year. Lacking that money, the City of Springfield is considering either building a second lake or damming part of the nearby Sangamon River to insure itself a future water supply.
Virtually every lake in Illinois suffers from the same disease. Lake Paradise in Mattoon became so clogged that the city had to build a new lake in 1958. The Illinois Department of Agriculture, in cooperation with other state and federal agencies, has begun a project to dredge 6,700 tons of sediment from the old lake and deposit it on nearby test plots—recycling topsoil, in effect—to see if it will provide a useful medium for crop production. (Such sediment may harbor unhealthy doses of contaminants such as heavy metals.) Even if the experiment proves a success, however, it seems unlikely that mining old lakes will ever be an economical cure for sedimentation. Nature will always be able to move dirt much more cheaply than bulldozers can.
Sediment is more than just an expensive nuisance, however. In sufficient quantities, and over sufficient periods of time, sediment is as fatal as any poison to aquatic life. The amount of sediment suspended in water is reflected by the degree of the water's turbidity, or cloudiness. Years ago, turbidity was measured by submerging a white plate beneath the surface and measuring in inches the depth at which it disappeared from view. Today turbidity is measured in Jackson Turbidity Units (JTUs), using a gauge that operates on much the same principle. Today, most Illinois waterways show readings of from 20 to 100 JTUs, depending on the season and other factors.
Such concentrations of silt have many unhappy effects on aquatic ecosystems. For example, aquatic food chains depend ultimately on photosynthesis, by which green plants transform light and carbon dioxide into food. The zone into which life-giving light penetrates is called the euphotic zone. In a clear lake, the euphotic zone may extend all the way to the bottom. But in a lake showing a turbidity of 50 JTUs, light scarcely penetrates the clouds of suspended sediment at all; one study of such waters showed that only 6.3 percent of the light striking the surface was visible at a depth of only four inches. As a result, the primary production of food is drastically curtailed, with populations of plankton in some instances dropping by as much as 80 percent compared to clear water. Larger plants suffer too, which in turn cheats the populations of snails, insects and small fish which feed upon, breed under or take shelter beneath them.
In extreme concentrations, silt clogs the gills of fish and mussels. It obscures vision underwater, thus crippling those species, like the bass (which can't spawn in waters more turbid than 84 JTUs) and the bluegill, which uses visual cues in feeding and mating. Turbidity harms fishing too. In the underwater gloom, fish can't see anglers' lures; game fish populations thin out or die off completely, and their place is taken by more tolerant bottom dwellers who rely on smell and touch for feeding, such as the carp.
When it settles, sediment obliterates natural underwater topography, covering a variegated habitat with a featureless blanket of ooze. Sediment covers sand and gravel beds vital to fish spawning cycles and provides a root medium too unstable to support aquatic plants. As plant growth slows, the amount of dissolved oxygen in the water drops. In extreme cases, the final inheritor of such a lake are the anaerobic or nonoxygen-using bacteria which feed on decaying organic matter, emitting putrid hydrogen sulfide gas as proof of their presence.
But sediment must be condemned not only for the damage it causes but for the damage it brings. When soil washes away from a farm field (or to a lesser extent, from an urban building site or a suburban lawn), it carries with it an assortment of chemicals—weed killers, fertilizers, pesticides—which have been applied to the soil and which become dissolved in runoff water or attached (adsorbed) to the soil particles themselves.
These chemical hitchhikers worry state environmental officials as much as the sediment itself, particularly as the amounts of such chemicals in use (especially on farms) continue to increase. Their concern explains the otherwise anomalous fact that the IEPA, in carrying out its part of the federally mandated water quality planning process, took the lead in farm erosion-control planning in Illinois in the late 1970s. When the IEPA surveyed 353 Illinois lakes in 1978, it found that all of them were eutrophic (a word that means literally "rich in food"). An excess of basic plant nutrients such as nitrogen and phosphorus prompts excessive growth of aquatic plants, including algae. This process is a part of any lake's natural history, but humans can speed it up materially. Ironically, the high turbidity of many Illinois lakes is a saving fault in that it blocks light which would feed even more vigorous plant growth.
Nitrogen and phosphorus enter such lakes in various forms from farm fields, where they are originally applied as fertilizer. But fields are not the only source of these nutrients; septic fields, sewage, even the feces of waterfowl contribute their share, and it is unclear how much fertilizers contribute to eutrophication of Illinois lakes.
Still, farm fields are presumed to be a major source of such nutrients, and eroded soil particles often are the means by which they migrate. For example, phosphorus is not very soluble in water. (Some tests have shown that less than 1 percent of phosphorus applied as fertilizer leaves a field in runoff water.) But it does attach itself to fine soil particles; indeed, because most eroded soils tend to be made up of these easily transportable particles, the amount of phosphorus found in sediment can be as much as three to four times higher than that in the farm soil from which it came. In fact, there is some concern that improved erosion control might actually increase phosphorus contamination in some watersheds. There would be less soil washed away, but the soil that was washed away would carry a higher proportion of adsorbed phosphorus, especially if farmers were to switch to certain reduced tillage methods which require that fertilizers be applied to the surface rather than incorporated into the soil.
Pesticides tend to be less easily transported than fertilizers, but they too have been showing up in Illinois lakes. (Of the 23 pesticides in common use in Illinois, 17 move via adsorption to soil particles). In the past, wide use of long-lived organochlorine compounds such as DDT led to much contamination. In 1977 the fish in 14 lakes and rivers were found to harbor concentrations of organochlorines (mainly Dieldrin) that were higher than the minimums considered safe by the U.S. Food and Drug Administration, even though sales of Dieldrin had been halted several years earlier.
Most of the pesticides in use by Illinois farmers today are the relatively short-lived organophosphate type which break down chemically in water within a week or two. Still, they remain potent killers in concentrated doses. In 1980, 13 of the recorded 23 fish kills in Illinois were traced to farm chemicals, usually pesticides washed into adjacent water supplies by a heavy rain. Prior to 1976, most fish kills in the state were traceable to industrial rather than agricultural sources.
Cleaning up this muddy mess is difficult for two reasons. For one thing, sediment can be a pollutant out of water as well as in it. Dredging a lake bottom, for example, merely puts the accumulated muck (including its bevy of chemicals) back into circulation unless extraordinary steps are taken to seal it off from its surrounding environment. Occasionally even stirring up a silty lake bottom will, by making organic matter available, trigger a feeding spree by microorganisms sufficiently massive to deplete the dissolved oxygen in the water, rendering it lifeless.
Beyond the environmental risks is the problem of costs. In 1967 the Illinois Department of Transportation reported that cleaning roadside ditches alone cost county and state highway departments $6.3 million a year. The U.S. Army Corps of Engineers spends more than a million dollars a year scraping mud off the bottom of the Illinois River to keep it open to barge traffic. The IEPA has estimated that the cost of building new lakes and dredging old ones to replace the 8,300 acre-feet of water storage capacity the state loses each year is nearly $21 million. But for all their expense, such steps are merely palliatives, not cures.
Stopping the problem at its source means implementing soil conservation practices which are also far from cheap. In 1979, the state's district soil conservationists were surveyed by the IEPA and reported that the minimum additional cost of conservation would be more than $1.3 billion—a figure which did not include the costs of changed tillage practices, maintenance and reduced income from land taken out of cultivation.
Many farmers, in short, continue to let their land erode because they simply cannot afford to do otherwise. Others, however, don't yet realize that erosion is a problem on their land, while others who do underestimate its effects.
In a great many cases, however, erosion is allowed because in the short-term it is economical. The University of Illinois has estimated that roughly 55 percent of the farmland in Illinois is not owned by the people who farm it. Illinois has more of its farmland in absentee ownership than any other state and has had for a long time. Some owners hold farmland as a long-term capital investment, while others, including many retired farmers and farm widows, rely on it as an income producer. Still others buy it for a quick resale to developers.
Landlords who hold farmland for the sake of short-term return often care little about the long-term effects of cultivation practices on soil fertility. Conservation improvements such as terraces are expensive to build and take many years to pay for themselves. Tenants likewise have little incentive to husband the land. Contracts are typically written for a year or two at most, and since the tenant's income rises and falls with yields, he is perhaps even less eager than the landlord to sacrifice current income for future fertility. While some reduced tillage methods can cut erosion without sacrificing yields, they generally require more diligent management and different equipment than traditional methods—a further disincentive to save soil. It is a measure of the indifference with which soil conservation has been held that standard farmland rental contracts contain no provisions pertaining to erosion control, beyond a ritual caution to tenants to not destroy conservation improvements already in place.
Economics and erosion are linked in other ways. It is no coincidence that the spurt in the rate of erosion in Illinois in the mid-1970s coincided with the lifting of government land set-aside programs and the expansion of the export trade in grain. Just as farmers (especially Illinois farmers) have come to rely on exports to absorb excess production and maintain prices, so the nation has come to rely on farm exports to offset U.S. purchases abroad, particularly of petroleum. Critics increasingly liken the U.S. to a Third World nation, trading away its natural resources, in this case the soils of the Midwest, to pay for its imports.
Of the many costs of soil erosion—vitiated wildlife habitat, shrunken water reservoirs, clogged drainage systems, reduced recreation, contaminated water—none is more worrisome in the long run than the loss of the agricultural productivity of the land itself. State officials calculate that farm soil productivity has dropped at an average annual rate of 0.022 percent, or 11 percent over the last century. The cost at current prices of the crops that were not harvested in Illinois as a result of this drop in productivity has been estimated to be $110 million a year.
This decline seems miniscule. But productivity losses would be much higher were it not for the massive infusions of artificial fertilizers. (Land planted in corn without fertilizers continuously since 1876 at the University of Illinois’s Morrow Plots at Urbana yields roughly 50 bushels per acre on average. That is evidence of the remarkable fertility of the old Grand Prairie soils. But in 1979—admittedly a record year—average yield in the rest of Champaign County was 144 bushels, and the average yield over the last four years is 119 bushels.) Evidence indicates that fertilizer application, particularly of nitrogen, may have reached a point of diminishing returns. Although it is a rough correlation at best, corn yields in Illinois went up by only 10 percent from 1975 to 1979, while nitrogen application increased by 25 percent during the same period.
Experts predict that if erosion is controlled, corn yields in Illinois should increase by 20 bushels per acre or about 20 percent over the next 30 years. Such increases will be vital if the state's farmers are to meet increased global demand and, more importantly, keep pace with the spiralling costs of farming. If erosion is not controlled, yields will still increase, but very slowly—much more slowly than anticipated increases in production costs.
Illinois farmers have long since passed the point at which they drew on the soil itself for nutrients. And there remains the chance that a technological breakthrough will obviate the further productivity losses caused by erosion, much as the exhaustion of the soil after the first century of intensive farming was remedied by massive doses of medicinal fertilizers. Among the possibilities are crop varieties that make more efficient use of nutrients, such as corn plants which (like soybeans) would capture their own nitrogen from the air.
Indeed, the question is sometimes asked: Do farmers need soil at all? After all, soil today has been reduced to a largely mechanical function. It supports the plants physically, provides a medium through which exchanges of water and chemicals may take place, and acts as a reservoir of sorts upon which plants may draw during dry periods.
Unfortunately, many Illinois top-soils are underlain by subsoils which are much less fitted for these apparently mundane tasks than the topsoils. Clay subsoils, for example, are far less porous than loamy topsoils; roots find it hard to penetrate such soils, which are sticky when wet and when dry harden into something very like concrete. Sandy soils, in contrast, are too porous, and plants parch during dry times because rainfall drains away too rapidly. Topsoil may not be able to sustain super-high yields without the aid of fertilizers (although it could come closer, given proper restorative treatment), but its role in the production of food is still vital.
In the 1930s, there were a few instances in southern Illinois of land being "voided," or abandoned because of severe erosion damage. The grandchildren of those farmers face no such immediate disaster. Even drained by erosion, most Illinois soils remain marvelously productive. It is economic erosion that most threatens Illinois farms of the '80s. The difference between profit and loss is usually only a few bushels per acre. Anything which cheats a farmer of those crucial bushels—a string of bad weather years, further escalations in the price of fertilizer or fuel, and so on—can so reduce his return on his land that he can no longer economically justify farming it, even though the actual reductions in yield may be small. One needn't destroy land to render it unfarmable. One need only make it unprofitable. ●
Illinois: The Sediment State
Sedimentation as a process is hardly new to Illinois, even if it has been a problem only in the last hundred years. Beneath the state's rather bland surface lie dozens of layers of rock which some geologists have described as a nest of spoons. Science writer Alan Anderson Jr. explains that most of Illinois is one vast basin that has been alternately raised and lowered past sea level perhaps fifty times in all. "When it was raised, water and wind eroded the surface; when it was lowered, it gathered sand and silt brought by mighty rivers [some from as far away as Canada] or limestone and dolomite precipitated from sea water." The result is that the Illinois basin "has been filled over the last half-billion years by as much as three miles of sediment" [emphasis added]. Compared to such brobdingnagian earth-moving feats, the loss of a few inches of topsoil seems scant cause for worry.
But the same physical characteristics that make topsoil susceptible to erosion in the first place complicate the life cycles of the lakes into which it settles. Fine particles are easily uprooted, and once uprooted they stay suspended in water for long periods. In shallow lakes, bottom-feeding fish such as the carp roil the sediment and keep it in suspension nearly constantly; when a carp wriggles away from the shallows, it leaves an incongruous plume of "dust" behind him.
Barge tugs are rather like giant carp in this respect. Researchers have discovered that the churning propellers of such boats so disturb sediments on the Illinois River that turbidity skyrockets after a boat passes, jumping from 70 Jackson Turbidity Units (a standard measure of water's cloudiness) to as many as 150 JTUs. It takes roughly two-and-a-half hours for that sediment to settle back to the bottom again. A steady traffic of barges, then, would seem to condemn the creatures of the Illinois to perpetual gloom.
When it is allowed to settle, sediment acts peculiarly. In some places along the Illinois River, for example, sediment deposits are more souplike than solid, and there are reports of people drowning in them. And when a lake with an irregularly shaped bottom fills up, the sediment does not distribute itself equally. Instead, the deeper spots fill fastest and deepest because the waters above them deliver a heavier total load of sediment. As the lake continues to fill, the rate of sedimentation slows because the volume of sediment available diminishes as the water volume diminishes. In time, sedimentation reduces the most antic bottom topography to an almost perfectly flat plain.
Incidentally, while most sediment originates on agricultural land, some of the worst erosion damage occurs in cities. "In the total number of tons lost, erosion from urban sites isn't great," explains Ron Gerhardt, district conservationist in Cook County. "But the tons lost per acre in urban areas can be up to 100 times higher than from farmland, because the land is left bare for longer periods—up to two years instead of a few weeks." ●
The Illinois River: An Ecological Disaster
The effects of the cycle of sedimentation and ruin can be seen most vividly along the Illinois River. What has happened to the Illinois system—the river plus backwater lakes, sloughs and flood plains which originally totaled roughly 400,000 acres—since the turn of the century is an ecological catastrophe. Seventy-five years ago the lower Illinois attracted swimmers and boaters from across the state. Summer resorts flourished, and each fall hunters swarmed into the Illinois valley from many states in search of migratory waterfowl so numerous that even a mediocre shot could bag up to 100 ducks a day. Fishing was equally good, and not just for the sportsman. In the early 1900s the 15 million pounds of fish, mussel and turtle taken from the river made it the second richest commercial freshwater fishery in the U.S. after the Great Lakes.
Within a generation, however, half the backwater lakes along the river were drained and put under the plow. Pollution from cities upstream (mainly Chicago) turned the Illinois into a sewer; by 1920 the river was dead, devoid of dissolved oxygen, as far south as Chillicothe.
Improved sewage treatment upstream gradually cleaned the water, and the river ecosystem revived through the 1940s and 50s, only to fall victim to a second enemy—sediment. The Illinois is truly a "river gently flowing," as the state song says; its vertical drop from source to mouth is very modest, and flow is further slowed by a series of navigation dams. Since the ability of moving water to carry sediment varies with its speed, the lethargic Illinois drops most of its sediment in and (when its waters overflow in flood) along its channel.
Soil pollution, in the opinion of Frank Bellrose of the Illinois Natural History Survey, has proved to be even worse than sewage or chemical pollution because "it's more insidious and it's more persistent." The backwater lakes that were not already drained have been gradually strangled by sediment from the farms of the rich Illinois valley.
The effects of sedimentation on the river's ecosystem are dire. Except for carp and buffalo, most of the fish are gone, as are the mussels. Waterfowl populations have survived somewhat better, but even the migratory flocks are finding the valley a less congenial stopover; the weedy biomass of the river's backwater lakes—a sizable proportion of which is duck food—averaged more than a ton per acre in the 1920s but by the 60s many of them yielded none at all. They are half to three-fourths filled with mud, and in the worst of them it is hard to find enough water to float a canoe.
Researchers from the Illinois Natural History Survey and its predecessor agencies have been studying the Illinois system since 1890. Their investigation has taken on the nature of an autopsy. They have learned that the rate of sedimentation has been increasing in recent years, going from an average of 0.67 inches per year prior to 1965 to 1.21 inches per year since then. A study by the state's Water Survey reveals that 25 million tons of soil pour into the Illinois each year. Of this, 14 million tons remain in the river as sediment—a statistic Bellrose characterizes as "startling."
Peoria Lake will be a mud flat in 50 years; the 3,000-acre Lake Chautauqua federal wildlife preserve will disappear in a century. The average depth of the remaining 70,000 acres of backwater lakes is now only two feet, and scientists fear that because heavy rains kept river waters in the lakes for long periods, 1981 may prove to be one of the worst years on record for sedimentation. In time the lakes will become mosquito-ridden bottomland forests populated by willows, cottonwoods and soft maples. ●