EROSION AND SEDIMENTATION


From The Changing Illinois Environment: Critical Trends, Volume 2: Water Resources , Technical Report of the Critical Trends Assessment Project

INTRODUCTION

Erosion and sedimentation have been important issues in Illinois for a very long time. In the early stages, the concern was primarily with the loss of soil productivity for agricultural crops, with less emphasis on off-site environmental impacts. In southern Illinois, where most of the state's population lived at the turn of the century, topsoil from the unglaciated Shawnee Hills areas was severely eroded by the early 1900s. Some areas were "... abandoned because so much of the surface soil had been washed away, and there were so many gullies that further cultivation was unprofitable." (Walker, 1984). Much of the present day Shawnee Forest consists of severely eroded areas that were purchased by the federal government and planted with trees.

In recent years most of the concern around erosion and sedimentation has focused on its impact on environmental quality. The Illinois Water Quality Management Plan, developed after extensive research and public review, stated that "The most severe agricultural related water quality problem is soil erosion and sedimentation" (IEPA, 1982). Similarly, the Illinois State Water Plan, developed in 1984 after three years of public review and discussion of all water resources issues in the state, identified erosion and sediment control as the number one water resources issue. The plan stated that "Excessive soil erosion on 9.6 million acres of Illinois farmland is threatening their productive capacity, degrading water quality, accelerating eutrophication of reservoirs, silting streams, and degrading fish and wildlife habitat" (Illinois State Water Plan Task Force, 1984).

More recently, the discussion of the erosion and sedimentation issue has focused on the impact of continuing sedimentation on the Illinois River valley. Several conferences have been organized in recent years to discuss the issue. After the first Illinois River Conference in 1987, the Governor requested the State Water Plan Task Force to review the conference proceedings, Management of the Illinois River System: The 1990s and Beyond (WRC, 1989), and make recommendations for actions that could be implemented. As a result, the State Water Plan Task Force prepared the Illinois River Action Plan (1987). The report ranked "soil erosion and siltation" as the top priority problem for the Illinois River and stated that "sedimentation, today's major pollutant of our nation's agricultural waterways, is the primary obstacle in preserving some semblance of the historic Illinois River for future generations."

After the first Governor's Conference on the Management of the Illinois River System in 1987, Bill Mathis and Glenn Stout summarized the discussions: "Most of the problems uppermost on the minds of the participants included significant problems with soil erosion and siltation. All groups recognized that soil erosion and siltation from land use practices threatened the Illinois River, its backwater lakes, and associated biota" (Mathis and Stout, 1987).

At the 1991 Governor's Conference on the Management of the Illinois River System, Colonel James Craig, Commander of the St. Louis District of the U.S. Army Corps of Engineers, stated that "The greatest challenge facing us on this portion of the Illinois River is sedimentation. Sedimentation clogs the navigation channel, and increases turbidity of the river. In the last 35 years, we have had to dredge over 14 million cubic yards of sediment from the river" (Craig, 1991). Several other public officials and scientists have expressed similar messages over the years.

Based on the review of the concerns and assessment of natural resources and environmental agencies in the state, soil erosion and sedimentation are unquestionably important natural resources and environmental issues in Illinois. It is generally agreed that the erosion rate is above the tolerable limit and that the off-site impacts of the eroded soil must be addressed by the agricultural and environmental communities of the state. A major contribution of the Critical Trends Analysis project (CTAP) will be to stress the importance of the issue, point out what has been done to control the problem, indicate any measurable changes over time, identify where the problem is most serious, and identify the problem areas that must be addressed in the future.

As a product of CTAP, two reports on soil erosion and sedimentation have been prepared. The first (Akanbi and Demissie, 1993) is a technical report that contains available data on the subject from the Illinois State Water Survey (ISWS), the U.S. Geological Survey (USGS), and the Illinois Environ-mental Protection Agency (IEPA). The data were compiled and analyzed, including regional sediment yields and temporal sediment concentration trends.

This report, the second on erosion and sedimentation, discusses the issue in a broader perspective and pre- sents results from previous studies conducted at the ISWS: Erosion and Sedimentation in the Illinois River Basin (Demissie et al., 1992); Peoria Lake Sediment Investigation (Demissie and Bhowmik, 1986); Conceptual Models of Erosion and Sedimentation in Illinois (Bhowmik et al., 1984); Sediment Loads of Illinois Streams and Rivers (Bhowmik et al., 1986); Sedimentation and Hydrologic Processes in Lake Decatur and Its Watershed (Fitzpatrick et al., 1987); and Sedimentation Investigation of Lake Springfield, Springfield, Illinois (Fitzpatrick et al., 1985). Infor- mation from these reports and the Akanbi and Demissie report (1993) has been used extensively for this report.

BACKGROUND

Erosion and sedimentation are critical environmental and economic issues that have been discussed extensively in the literature for generations. Civilizations and productive environments have collapsed because of the mismanagement of land, and excessive erosion has rendered vast areas useless for sustainable agriculture. Excessive sedimentation has also obliterated highly productive and important natural aquatic environments. What must be realized, however, is that soil erosion and the resulting deposition of eroded soil in aquatic environments (sedimentation) are natural processes. They have been taking place since the creation of the earth and are responsible for gradually reshaping the surface of the earth over geologic times.

Erosion and sedimentation are inseparable. Wherever there is erosion there will be sedimentation, and wherever there is sedimentation there must have been erosion somewhere else. When erosion and sedimentation rates are within tolerable limits, changes are gradual and not noticeable. However, when human activities such as agriculture, construction, mining, timber harvesting, and water course management drastically accelerate the erosion and sedimentation processes, then economic and environmental impacts are significant and long lasting. In the 1984 State of the World report, Brown estimated that 4 billion tons of soil are washed into the oceans annually worldwide (Brown, 1984). Some regions are impacted more seriously than others. In the United States, the issue of soil erosion and its impact on agricultural productivity and the environment have been discussed and studied widely. Despite significant time and resources spent to control soil erosion and reduce its impacts in the United States, the problem persists and might even be getting worse in some areas. William K. Reilly, former president of the Conservation Foundation and former head of the U.S. Environmental Protection Agency (USEPA) summarized the issue of soil erosion in the United States succinctly and eloquently in Sandras Batie's book, Soil Erosion: Crisis in America's Croplands? (1984):

Soil erosion is not yet a crisis in this country. The products of American farms still feed a good part of the world, and will continue to do so for some time. But despite 50 years of federal efforts and billions of dollars expended to conserve productive soils, erosion persists as one of this country's major conservation problems. Erosion not only robs farmland of its fertility, it also seriously pollutes the nation's waterways. It may even have accelerated as farmers responded during the past decade to economic opportunities and pressures by planting more cropland, including marginally productive lands and lands prone to erosion, and abandoning conservation measures. Ironically, most Americans believe our soil erosion problem was resolved during the 1930s when severe droughts and dust storms swept across the prairies, and midwestern soil accumulated on windowsills of the Capitol in Washington, DC.

For this generation, with its new farm technology and its dim recollection of the social and economic catastrophe to which soil erosion contributed in the 1930s, we need to put this issue on the nation's agenda. This book is part of the effort to do that. If Americans do not take seriously the accumulating evidence about the extent and consequences of erosion, the country's agricultural future may be undermined, perhaps not this decade or next, but sometime early in the twenty-first century. If this happens, then I believe decision makers of today would have violated a trust they hold for future generations of Americans.

Even though soil erosion has not reached a crisis level, it is recognized as a major environmental, resource, and economic problem. Many federal and state programs have attempted to manage it, but the problem persists. More efforts will be launched to control soil erosion, but their success will depend on the proper understanding of the problem and implementation of programs that work. The main contributions of the scientific community in such an effort are to evaluate past practices, to identify the key problem areas, and to recommend solutions that work. Significant resources have been spent on soil erosion control programs in the past, but the results are not encouraging. Soil erosion is still a major problem, and the resulting sedimentation is significantly impacting aquatic environments and water resource facilities.

The negative impacts of erosion and sedimentation are generally grouped into on-site and off-site impacts (Clark et al., 1985). The on-site impacts, those that occur at the erosion site, generally involve the loss of fertile topsoil on the farm, in excess of the tolerable limit that will not reduce the productivity of the land. There is total unanimity on the concept of controlling soil erosion on the farm to retain the fertile soil and maintain its productivity. The controversy is generally on how to do it and who should pay for it.

The other major on-site impact relates to streambank erosion. General agreement holds that excessive streambank erosion, whether in rural or urban areas, is not good for the property owners and should be controlled. Even though certain land use practices and hydrologic and hydraulic modifications could be directly linked to the erosion problem, mitigating the problem has not been easy. Federal and state agencies have spent billions of dollars on streambank erosion projects without solving the problem. Again the controversy is how to manage it and who should pay for it. In most cases, little attention goes to preventing the problem in the first place, even though this might be the least expensive and most enduring solution.

Off-site impacts of erosion and sedimentation have moved into the forefront in the last two decades. These impacts are caused by the transport and deposition of eroded soil to places other than the erosion sites. These environments are generally associated either with moving or standing water. Most streams, rivers, lakes, reservoirs, and wetlands are impacted by the transport or deposition of excessive sediment. The annual off-site damage caused by sediment in the United States is estimated to exceed $6 billion, of which more than one third is attributed to erosion from cropland (Clark et al., 1985).

Excessive sediment movement and deposition can cause rivers, reservoirs, lakes, wetlands, navigation channels, drainage ditches, and canals to silt up and lose their capacity to carry and store water. Correcting these problems is generally very expensive. It costs millions of dollars to dredge even small stretches of navigation channels and water supply reservoirs.

Other off-site impacts of erosion and sedimentation are related to water quality and aquatic habitats. Sediment is generally included in what is referred to as "nonpoint source" or "diffuse" pollution. In evaluating the quality of the nation's waters, the USEPA concluded that "...the two leading pollutants affecting the nation's rivers and streams are predominantly of diffuse origin. These two pollutants are siltation--the smothering of streambeds by sediments, usually from soil erosion--and nutrients" (USEPA, 1990). The nationwide USEPA survey is summarized in figures 1 and 2, which rank the top ten causes and sources of pollution for rivers, streams, lakes, and reservoirs. Siltation (sedimentation) is identified as the number one pollutant for rivers and streams and the number two pollutant for lakes and reservoirs. Illinois reported that 94 percent of its impaired rivers and streams and 100 percent of its impaired lakes and reservoirs are affected by siltation (USEPA, 1990).

Even though other land-use practices contribute significantly, especially in localized situations, agriculture is believed to be the main source of the erosion and sedimentation problem. In Illinois, with more than 65 percent of the land used for crop production, agriculture was identified by the IEPA as the source of pollution for 99 percent of the impaired rivers, streams, and lakes.

Therefore, in general it can be concluded that erosion and sedimentation are global problems, despite significant attempts to control erosion and reduce its impact on the productivity of agricultural lands and on the environment. Even with all the efforts and enormous resources spent to control it, a more scientific basis for erosion control and mitigation of its impacts remains to be developed. The monitoring, analysis, and evaluation of soil erosion rates, sediment transport, and sedimentation rates must be the basis for scientific programs and policies that can eventually control the problem.

EROSION AND SEDIMENTATION ISSUES IN ILLINOIS

The following sections will present brief discussions of the major erosion and sediment issues as they apply to Illinois.

Watershed Erosion

Watershed erosion refers primarily to sheet, rill, and gully erosion from agricultural and nonagricultural areas. It does not include streambank erosion and erosion from construction sites and mining operations. Since more than 65 percent of the Illinois land surface is used for agriculture, watershed erosion is significantly influenced by cultural practices. The annual gross soil erosion from croplands is estimated to be 158 million tons, nearly 90 percent of the total gross soil erosion of 180 million tons for the state (IEPA, 1982). Erosion from nonagricultural lands therefore accounts for about 10 percent of the total erosion in the state. In relation to overall watershed erosion from croplands, most of erosion occurs within a small portion of the watershed. These "critical erosion areas" generate disproportionate amounts of sediment.

The results of centuries of erosion on Illinois topsoil are summarized in figure 3, where its present day thickness is compared to what is estimated to have been its original thickness. With the exception of the Illinois, Mississippi, and Ohio River valleys, which have extensive sediment buildup, the whole state has lost 2 to 9 inches. The most severe erosion occurred in southern Illinois, where the hilly and highly erosive area was settled and farmed earlier than the rest of the state. Erosion has also been significant in the western part of the state, where 6 to 7 inches of topsoil has been lost. The central and northeastern parts of the state have fared better with 2 to 4 inches of erosion.

In Illinois most watershed erosion control activities for agricultural lands are carried out by the Illinois Department of Agriculture through the Soil and Water Conservation Districts (SWCD). "T by 2000" is a major Illinois initiative to reduce erosion from agricultural lands to tolerable soil loss limits ("T") by the year 2000. "T" is defined as the maximum average annual soil loss in tons per acre per year that can be tolerated by the soil and still sustain production into the future (Illinois Department of Agriculture, 1991). Nichols (1989) summarized the soil erosion control programs of the Illinois Department of Agriculture in the Illinois River basin.

The department allocates cost-share monies for erosion control projects through two programs: the Conservation Practice Program (CPP) and the Watershed Land Treatment Program (WLTP). The CPP encourages soil conservation practices by providing cost-share money for projects such as terracing, grassed waterways, grading, and sediment detention structures. The WLTP deals primarily with agricultural areas identified as critically eroding. Nichols identified 3.8 million acres of agricultural land in 36 SWCDs, primarily within the Illinois River basin, that are eroding at rates greater than the "T" values. He estimated that remediation will require the implementation of conservation practices for 4.5 million acres of agricultural lands at a cost of $327 million. The department spent nearly $6 million from 1986 to 1989 from the "Build Illinois" fund for erosion control in the region. In 1990, $0.8 million from the General Revenue Fund was allocated for erosion control within the same area. Thus, some progress is being made to control erosion from agricultural lands, but much more needs to be done. Of the total estimated cost of $327 million to control agricultural erosion within the watershed, roughly $7 million was expended in five years. Full realization of the "T by 2000" program through state and federal funding will go a long way toward controlling erosion and sedimentation problems in Illinois.

Streambank Erosion

Illinois has an estimated 13,200 miles of streams. Most streams experience some form of bank erosion. In cases where vegetation has been removed from streambanks, bank erosion is excessive. Many channelization projects and river crossing structures such as bridges tend to increase the streambank erosion potential. In attempts to quantify the percent of a stream's sediment load that originates from bank erosion, different investigators have reported 20 to 80 percent. The actual value will depend on the local conditions for a particular stream. In any case, streambank erosion is believed to be a major contributor of sediment to streams in the Illinois River basin.

Many streambank stabilization projects have been initiated in the Illinois River basin, primarily by local governments. The Illinois Department of Conservation promotes several demonstration projects using vegetation as the stabilizing agent although no statewide or basinwide control programs exist in Illinois. Realizing the significance of the problem and the fact that sedimentation problems will not be solved unless excessive streambank erosion is controlled in the basin, a comprehensive streambank erosion control program is needed. The program should identify major streambank erosion areas throughout the watershed and quantify how many stream miles are eroding at a significant rate. The types of streambank failures and the suspected causes also should be documented. This is important because all streambank erosion is not of the same type or initiated by the same cause. Once the locations, types, and causes of streambank erosion in the basin have been identified, appropriate streambank stabilization techniques can be recommended. Most of the streambank stabilization techniques were outlined in a report to Congress by the U.S. Army Corps of Engineers in 1981 (USACOE, 1981). Instream Sediment Transport

Only a percentage of the soil particles eroded from a watershed reach the stream system and are transported downstream from the erosion site. The ratio of the soil reaching the stream network to the total erosion in the watershed is defined as the delivery ratio. The delivery ratio depends on many physical and geomorphic fac-tors, although the drainage area is one of the most important. It can vary from as low as 3 percent for large watersheds to over 95 percent for small watersheds. To understand the off-site environmental and ecological impacts of soil erosion and sedimentation, it is essential to determine how much of the eroded soil reaches the waterways.

Since the methods and techniques used to quantify erosion and delivery ratios are generally empirical and highly unreliable, it is important to measure the actual sediment being transported by streams. Monitoring of the instream sediment load over a long period of time is the most reliable method of quantifying the amount of soil that moves from one place to another. Monitoring can detect changes in erosion rates, transport capabilities, impacts of hydrologic changes in the watershed, and effectiveness of land management practices. However, because of the expense of collections, in-stream sediment load data are not widely available. When they are available, they are of short duration.

Three Illinois agencies, the ISWS, the USGS, and the IEPA, have been collecting instream sediment data over the last 20 years. The distribution, the number of stations where data have been collected in Illinois, and the length of record at the stations are shown in figure 4. Data collection peaked in 1981 when the Water Survey initiated the Sediment Benchmark Network. However, the number of stations has declined since then, with 40 stations in operation in 1990. The major weakness in the database is the length of record at each station. The majority of the stations have data only for one to three years. The nature of the data has also changed over time. Other than the four USGS stations, sediment data are collected only weekly by the ISWS and the IEPA. This makes sediment load and budget calculations very difficult and unreliable.

However, even sediment concentration values alone provide valuable information on general trends in streams and rivers, such as those shown for selected streams in northern, central, and southern Illinois in figure 5. As shown in the figure, sediment concentrations in the Rock River in northern Illinois have been decreasing since 1980. They have been essentially constant in the Sangamon River in central Illinois, slightly decreasing in the Kaskaskia River in south-central Illinois, and slightly increasing in the Cache River in southern Illinois. As the length of these data increase, significant changes in sediment concentrations in streams can be detected more precisely. These changes in sediment concentrations can then be correlated to changes in land-use practice, climate change, or other natural or man-made modifications in the watersheds.

The other more significant use of instream sediment data is in calculating sediment yields from different watersheds. This information is not only important in detecting trends and evaluating the impacts of projects and land use changes, it is essential in designing reser-voirs and predicting sedimentation rates in streams, lakes, reservoirs, and wetlands. The rate of sedimentation is directly proportional to the amount of sediment in the flow. Comparison of regional sediment yields would identify the spatial pattern of sediment yield in the state.

All available sediment load data from 35 sediment monitoring stations were used to compute the spatial distribution and the annual sediment yields in Illinois. When the annual sediment yields were plotted against the annual water discharge, all the data fell into four groups (figure 6), implying that four equations could be used to calculate sediment yield from Illinois watersheds. The spatial distribution of sediment yield is shown in figure 7. The regions along the Illinois River and in west-central Illinois were found to be the highest sediment producing areas, along with some in southern Illinois. Northeastern and central Illinois show the least sediment yield.

Lake and Reservoir Sedimentation

One of the major off-site impacts of erosion is the eventual accumulation of the eroded soil in lakes and reservoirs, which become natural traps for sediment transported by streams and rivers. Because of the significant reductions in flow velocity, most of the sediment transported by streams and rivers settles out in lakes and impoundments. This continuous sediment accumulation gradually reduces the impoundments' depth and consequently their capacity to store water. For water supply lakes, the gradual loss of storage capacity is a major concern. In many cases, sediment has to be dredged at great expense. In some cases, significant effort is expended to reduce erosion rates in the watershed and subsequent sediment inflow to lakes. In very few cases, attempts are made to route the sediment past or around lakes. In all cases, the problem of sedimentation in lakes and reservoirs is expensive.

In Illinois, lake sedimentation has been a serious problem and has been investigated intensively. Sedimentation surveys are among the basic tools used to quantify sedimentation rates in order to project future water storage needs or shortages. They provide information on original and current lake capacity, sedimentation rates for different periods, sedimentation patterns, changes in depth of water, and sources of sediment. Information generated through sedimentation surveys is used in future water supply planning in areas such as projecting available storage capacities, lake dredging projects to increase storage capacities, and spillway and dam modifications when possible.

The Illinois State Water Survey has taken the issue of lake sedimentation as one of its primary missions since the early 1930s and has conducted more than 180 sedimentation surveys of more than 130 lakes in Illinois, compiling comprehensive lake sedimentation survey data. The Water Survey's data have been used by engineers, researchers, and planners throughout the world. The distribution of the surveys over time is shown in figure 8. Most were conducted in the 1950s, a time of acute concern for water supply storage during drought, although lake sedimentation surveys have been conducted fairly regularly since the 1940s, with more than ten surveys per decade.

Two good examples of water supply lakes with sedimentation problem are Lakes Decatur and Springfield in central Illinois. Both lakes are located in the Sangamon River basin, and serve as the sole source of drinking water for the cities of Decatur and Springfield, respectively. Lake Decatur was built in 1922, while Lake Springfield was built in 1934. The reduction of storage capacity over time for both lakes is shown in figure 9. Even though their capacity loss rates are in the range 0.3 to 0.5 percent per year, both lakes had to implement programs to restore storage capacities and reduce sedimentation rates. In 1956, the city of Decatur had to install gates on the spillway to increase the storage capacity of the lake. Springfield spent more than $10 million to dredge a small portion of the lake in 1988, and the city of Decatur is planning a major dredging operation in the near future. Both cities have been attempting to reduce the rate of erosion in the watershed and the delivery of sediment into the lakes.

The problems these two cities face in maintaining the water storage capacities of their lakes symbolize the problem of sedimentation as it relates to water supply needs. Some smaller cities have more serious problems with sedimentation and have had to rely on state and federal assistance to restore their lakes. As most lakes in Illinois age, the problem is expected to be more serious. The need for additional water storage capacities will require either more dredging or raising of spillway elevations. Under strict environmental regulations, these options are neither cheap nor feasible.

Sedimentation in the Illinois River Valley

As mentioned in the introduction, the Illinois River has become the focus of the erosion and sedimentation issue in Illinois in recent years. Because of its importance, the Illinois River represents the environmental status of Illinois streams, rivers, and lakes. Because of its central location and natural geomorphology, the river has been significantly impacted by erosion and sedimentation. Therefore, understanding the impacts of erosion and sedimentation on the Illinois River provides a very good indication of the problem in the state.

The following discussion is based on the results of a recent study on erosion and sedimentation in the Illinois River basin (Demissie et al., 1992). The Illinois River drains nearly half of the state. Many of the major streams in Illinois drain into it. In addition to its drainage, transportation, and commercial value, the Illinois River is an important ecological resource. With its numerous backwater lakes, wetlands, and floodplain forests, the Illinois River valley provides significant habitat for fish, waterfowl, birds, and other animals.

The Illinois River's environment has been subjected to many of the impacts associated with developments in the watershed, including waste discharges from urban areas, water level control for navigation, and sediment and chemical inflow from agricultural lands. The water quality of the river was severely degraded for several decades prior to the 1970s when environmental regulations were enacted to control pollutant discharges. Since then the quality of the river water has been gradually improving.

However, problems associated with erosion and sedimentation have not been improving and are recognized as the number one environmental problem in the Illinois River valley. The main sources of sediment to the Illinois River valley are watershed erosion, streambank erosion, and bluff erosion. The contribution of watershed erosion to the sedimentation problem in the Illinois River valley can be quantified by analyzing the sediment yields of tributary streams that drain into the valley. The contribution of bank erosion in the Illinois River valley and bluff erosion along the Illinois River are much more difficult to quantify at present because of the lack of data.

In an attempt to quantify the problem, sediment yields were calculated from tributary streams of the Illinois River based on suspended sediment load data collected by the USGS (Demissie et al., 1992). The sediment yield calculations were then used to construct an approximate sediment budget for the Illinois River valley. The calculations show that on the average, 13.8 million tons of sediment are delivered to the Illinois River valley annually. The average annual outflow of sediment from the Illinois River at Valley City is 5.6 million tons. This means that an average of 8.2 million tons of sediment are delivered from tributary streams and deposited in the Illinois River valley each year. The actual figure is expected to be higher than 8.2 million tons because of the contributions of bank and bluff erosion, which are not included in these calculations.

The temporal trends in sediment concentration and load in the Illinois River at Valley City (figure 10) have been decreasing since 1980. But the primary cause, at least for the sediment load, is the decreasing trend in streamflow over the same period.

Major areas impacted by sediment deposition in the Illinois River valley are backwater lakes, which had lost 20 to 100 percent of their capacities by the year 1990. The average capacity loss is 72 percent. Therefore most of the lakes, which are remnants of a much larger glacial river system that once occupied the Illinois River valley, have lost a large part of their capacities, and some of them have already filled completely with sediment.

The impact of sedimentation in the Illinois River valley has been clearly illustrated by the conditions in Peoria Lake over the years (Demissie and Bhowmik, 1986). Peoria Lake is the largest and deepest bottomland lake in the Illinois River valley. But a combination of accelerated erosion and hydraulic regulations has resulted in severe sedimentation rates in the bottomland lakes in recent decades. As of 1985, Peoria Lake had lost 68 percent of its 1903 capacity due to sedimentation. The average depth of the lake had been reduced from 8 to 2.6 feet. The reduction of lake capacity and depth over time is shown in figure 11.

The severity of the sedimentation in Peoria Lake is illustrated by figure 12, in which the 1903 and 1985 lake bed profiles are compared at four locations along the lake. As shown, much of the lake has filled with sediment. The sedimentation rate is higher in the upper lake than in the lower lake. Moreover, the lake is shallower in the upstream direction, so much of its upper end has filled entirely with sediment. The deeper channel through the lake is maintained for navigation.

The net result of sedimentation in Peoria Lake is the loss of the deeper parts of the lake. In figure 13, the portions of the lake deeper than 5 feet are compared for 1903 and 1985. In 1903 much of the lake would have been deeper than 5 feet under present day normal pool conditions, while in 1985 much of the lake was shallower than 5 feet, with only a narrow navigation channel through the middle. As sedimentation continues and the shallow flat areas start supporting vegetation, much of the lake will be transformed into a wetland that will be flooded regularly. The transformation of Peoria Lake into a narrow navigation channel with bordering wetlands and mudflats will not only reduce aesthetics but will also have negative impacts on recreation, real estate values, and tourism.

Soil erosion and sediment yield are related to land use practices in the watershed. In the Illinois River basin, 80 percent of the watershed is used for agriculture. Thus, an investigation of the land use practices, especially those of agriculture, should explain the high erosion and sedimentation rates in the Illinois River basin.

Agricultural acreage in the state and in the Illinois River watershed both increased slightly from 1925 to 1981 and then started to decline. The total acreage in Illinois varied from a low of 17.8 million acres in 1934 to a high of 23.9 million acres in 1980. For the Illinois River basin, the total acreage varied from a low of 11.7 million acres in 1934 to a high of 15.9 million acres in 1977. The Illinois River basin contains more than 60 percent of the agricultural acreage in the state. The percentage of agricultural lands in the basin as compared to the state's total has declined slightly since the 1930s.

The major crops harvested in the basin are corn, soybeans, wheat, oats, and hay. The changes in acreage for the leading Illinois crops are shown in figure 14. Corn acreage has been almost steady for the last 62 years (1925-1987), with more than 6 million acres harvested. On the other hand, soybean acreage has dramatically increased since the 1920s, from virtually no acreage in 1925 to nearly 6 million acres in 1987. At the same time, the acreage in grassy crops (wheat, oats, and hay) has declined in proportion to the increase in soybean acreage. Thus acreage of row crops (corn and soybeans) has significantly increased, while acreage for grassy crops has steadily decreased. This changing land use pattern and improvements in farm technology are generally assumed to be the major causes for the increased erosion and sedimentation in the Illinois River valley.

Sediment Quality

Releases of chemicals to the atmosphere, land surfaces, and surface and ground water occur continuously in both urban and agricultural surroundings. Once chemicals are released to the environment, they are inevitably transported and deposited at other locations by either air or water. Those transported through the atmosphere eventually end up in water bodies, while the final sink for these chemicals is either surface water impoundments or ground water. While groundwater contamination is limited to chemicals in the dissolved state, surface water contamination includes both dissolved and particulate chemicals. Most of the chemicals in surface waters have a strong affinity for finer sediment particles. Some of the chemicals are adsorbed (i.e., attached) onto the sediment particles, while others exist in particulate form mixed with the sediments.

The chemical characteristics of the different layers of lake sediments are very good indicators of the history of the environmental quality of their surroundings. For example, lake sediments clearly show the periods of atmospheric testing of nuclear weapons and periods of serious pollution by different chemicals.

Examples of chemical profiles in lake bed sediments from Peoria Lake are shown in figure 15, where the concentration of zinc and lead in the sediment are plotted against the depth of the sediment and the age of the sediment layers. The highest concentration of lead was in the late 1960s, while that for zinc was in the early 1950s. The concentrations of the two heavy metals in the sediment have been decreasing since the peak periods of deposition in the lake. The top and most recent sediment layer shows much lower concentrations of lead and zinc than the lower earlier layers. Similar patterns can also be observed for other chemicals in Illinois River sediments, and analysis of sediment core samples at other locations can provide the history of pollution in the region.

Most of the chemicals found in lake sediments are transported from source areas and upland watersheds, including urban and agricultural areas, through stream networks. Some chemicals are deposited directly into lakes from dry and wet atmospheric depositions and direct waste discharges. The transport and fate of chemicals that enter the stream network, lakes, and reservoirs are poorly understood and not very well documented in Illinois. There is, however, general consensus that the accumulation of chemicals in stream and lake sediments is a serious environmental problem, since it has a detrimental effect on water quality and aquatic biota.

SUMMARY AND CONCLUSIONS

Erosion and sedimentation have been important issues in Illinois for a very long time. In the early stages, the concern was primarily with the loss of soil productivity for agricultural crops, with less emphasis on off-site environmental impacts. In recent years most of the concern has focused on impacts on environmental quality and on continuing sedimentation on the Illinois River valley. The annual gross soil erosion from croplands has been estimated at 158 million tons, which is nearly 90 percent of the total annual gross soil erosion of 180 million tons for the state. Erosion from nonagricultural lands therefore accounts for about 10 percent of the total erosion in Illinois.

With the exception of the Illinois, Mississippi, and Ohio River valleys, which have extensive sediment buildup, 2 to 9 inches of Illinois topsoil has eroded. The most severe erosion took place in southern Illinois, where the hilly and highly erosive area was settled and farmed earlier than the rest of the state. Erosion has also been significant in the western part of the state, where 6 to 7 inches of topsoil has eroded. The central and northeastern parts of the state have fared better, with 2 to 4 inches of erosion.

The major impact of soil erosion is the eventual accumulation of the eroded soils in lakes and reservoirs. In Illinois, lake sedimentation has been a serious problem and has been investigated intensively. The Illinois State Water Survey has conducted more than 180 sedimentation surveys of more than 130 lakes in Illinois and has compiled comprehensive lake sedimentation survey data. Most of the lake surveys were conducted in the 1950s, a time of acute concern for water supply storage during the drought period. The gradual loss of water storage capacity in water supply lakes is a major concern. This has been illustrated by examining two water supply lakes with sedimentation problems in central Illinois: Lakes Decatur and Springfield. The storage capacity reductions over time for both lakes are in the range 0.3 to 0.5 percent per year. Nevertheless, both cities had to implement programs to restore storage capacities by reducing erosion rates in the watershed and the delivery of sediment into the lakes.

Three Illinois agencies, the ISWS, the USGS, and the IEPA, have been collecting instream sediment data over the last 20 years. Data collection peaked in 1981 when the Water Survey initiated the Sediment Benchmark Network. However, the number of stations has been declining since then, with a combined total of 40 stations operating in 1990. Analysis of the distribution of measured sediment concentrations for selected streams in northern, central, and southern Illinois shows that since 1980, sediment concentrations have been decreasing in the Rock River in northern Illinois, essentially constant in the Sangamon River in central Illinois, slightly decreasing in the Kaskaskia River in south-central Illinois, and slightly increasing in the Cache River in southern Illinois.

Analysis of data from 35 sediment monitoring stations for the annual sediment yields shows that four equations could be used to calculate sediment yield from Illinois watersheds. The spatial distribution of sediment yield shows that the regions along the Illinois River and in west-central Illinois are the highest sediment producing areas, along with some areas in southern Illinois. The northeastern and central sections of the state show the least sediment yield.

Because of its central location and natural geomorphology, the Illinois River has been significantly impacted by erosion and sedimentation. The Illinois River drains nearly half of the state, and many of the major streams in Illinois drain into it. The main sources of sediment to the Illinois River valley are watershed erosion, stream bank erosion, and bluff erosion. The sediment yield calculations show that on the average, 13.8 million tons of sediment are delivered to the Illinois River valley annually. But the average annual outflow of sediment from the Illinois River at Valley City is 5.6 million tons. Thus on the average, 8.2 million tons of sediment are delivered annually from tributary streams and deposited in the Illinois River valley.

The temporal trend analysis of sediment concentrations and load in the Illinois River at Valley City over the last ten years shows that they have been decreasing since 1980. But the primary cause, at least for the sediment load, is the decreasing trend in streamflow over the same period.

Major areas impacted by sediment deposition in the Illinois River valley are the backwater lakes. Sediment rate calculations show that the backwater lakes had lost 20 to 100 percent of their capacities by the year 1990, with an average capacity loss of 72 percent. The impact of sedimentation in the Illinois River valley has been clearly illustrated by the conditions in Peoria Lake over the years. As of 1985, Peoria Lake had lost 68 percent of its 1903 capacity due to sedimentation, and the average depth of the lake had been reduced from 8 to 2.6 feet.

Releases of chemicals to the atmosphere, land surfaces, and surface and ground waters occur continuously in both urban and agricultural surroundings. Most of the chemicals found in lake sediments are transported from source areas and upland watersheds, which include urban and agricultural areas, through stream networks. Chemical profiles for zinc and lead in lake sediments from Peoria Lake indicate that the highest concentration of lead was in the late 1960s, while that for zinc was in the early 1950s. The concentrations of the two heavy metals in the sediment have been decreasing since those peak periods of deposition. The top sediment layer shows much lower concentrations of lead and zinc than the lower layers, reflecting earlier periods. Similar patterns have also been observed for other chemicals in Illinois River sediments.

REFERENCES

Akanbi, A.A., and M. Demissie. 1994. Trends in Erosion and Sedimentation in Illinois. Illinois State Water Survey draft report, Champaign, IL.

Batie, S.S. 1984. Soil Erosion: Crisis in America's Croplands? The Conservation Foundation, Wash-ington, DC.

Bhowmik, N.G., J.R. Adams, A.P. Bonini, A.M. Klock, and M. Demissie. 1986. Sediment Loads of Illinois Streams and Rivers. Illinois State Water Survey Report of Investigations 106, Champaign, IL.

Bhowmik, N.G., M. Demissie, D.T. Soong, A. Klock, N.R. Black, D.L. Gross, T.W. Sipe, and P.G. Risser. 1984. Conceptual Models of Erosion and Sedimentation in Illinois. Vol. 1, Project Summary. Illinois State Water Survey and Illinois State Geological Survey Joint Report 1, Champaign, IL.

Brown, L.R. 1984. Conserving Soils. In State of the World, 1984, L.R. Brown (ed.), Norton & Co., New York.

Clark II, E.H., J.A. Haverkamp, and W. Chapman. 1985. Eroding Soils: The Off-Farm Impacts. The Con-servation Foundation, Washington, DC.

Craig, J.D. 1991. Management of the Illinois River. Proceedings of the 1991 Governor's Conference on the Management of the Illinois River System. Water Resources Center, University of Illinois, Urbana.

Demissie, M., and N.G. Bhowmik. 1986. Peoria Lake Sediment Investigation. Illinois State Water Survey Contract Report 371, Champaign, IL.

Demissie, M., L. Keefer, and R. Xia. 1992. Erosion and Sedimentation in the Illinois River Basin. ILENR/RE-WR- 92/04. Illinois Department of Energy and Natural Resources, Springfield, IL.

Fitzpatrick, W.P., W. Bogner, and N.G. Bhowmik. 1985. Sedimentation Investigation of Lake Springfield. Illinois State Water Survey Contract Report 363, Champaign, IL.

Fitzpatrick, W.P., W. Bogner, and N.G. Bhowmik. 1987. Sedimentation and Hydrologic Processes in Lake Decatur and Its Watershed. Illinois State Water Survey Report of Investigation 107, Champaign, IL.

Illinois Department of Agriculture. 1991. Annual Progress Report. Illinois Department of Agriculture, #7470/550, Springfield, IL.

Illinois Environmental Protection Agency. 1979. Water Quality Management Plan, Volume III: Nonpoint Sources of Pollution. Springfield, IL.

Illinois Environmental Protection Agency. 1982. Illinois Water Quality Management Plan. IEPA/WPC/82-012, Springfield, IL.

Illinois State Water Plan Task Force. 1984. Illinois State Water Plan, Critical Issues, Cross-Cutting Topics, Operating Issues. Illinois Department of Transportation, Springfield, IL.

Illinois State Water Plan Task Force. 1987. Illinois River Action Plan, Special Report No. 11. Illinois Department of Transportation, Springfield, IL.

Mathis, B., and G.E. Stout. 1987. Conference Summary and Suggestions for Action. Proceedings of the Governor's Conference on Management of the Illinois River System: The 1990s and Beyond. Special Report No. 16, Water Resources Center, University of Illinois, Urbana, IL.

Nichols, R.W. 1989. Controlling Soil Erosion in the Illinois River Basin.. Proceedings of Second Conference on the Management of the Illinois River System: The 1990s and Beyond. Special Report No. 18, Water Resources Center, University of Illinois, Urbana, IL.

U.S. Army Corps of Engineers (USACOE). 1981. Final Report to Congress. The Streambank Erosion Control Evaluation and Demonstration Act of 1974. Section 32, Public Law 93-251.

U.S. Environmental Protection Agency. 1990. National Water Quality Inventory. Office of Water, Washington, DC, EPA 440-4-90-003.

Walker, R.D. 1984. Historical Changes in Illinois Agriculture. Illinois Conference on Soil Conservation and Water Quality. ENR Doc. No. 84/02, pp. 1-17.

Water Resources Center (WRC). 1989. Proceedings of the Second Conference on the Management of the Illinois River System: the 1990s and Beyond. Special Report No. 18. Water Resources Center, University of Illinois, Urbana, IL.

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