Lake Decatur is the water supply reservoir for the City of Decatur. The reservoir was created in 1922 by constructing a dam to impound the flow of the Sangamon River. The dam was modified in 1956 to increase the maximum capacity of the lake to 28,000 acre-feet. The drainage area of the Sangamon River upstream of Decatur is 925 square miles and includes portions of seven counties in east-central Illinois. Lake Decatur has high concentrations of total dissolved solids and nitrates, and nitrate-N concentrations have been exceeding drinking water standards in recent years. This has created a serious situation for the drinking water supply of the City of Decatur, since nitrate-nitrogen (N) cannot be removed from finished drinking water through regular water purification processes. Nitrate-N concentrations in Lake Decatur have exceeded the Illinois Environmental Protection Agency (IEPA) drinking water standard of 10 milligrams per liter (mg/l) on occasions each year for the period between 1970 and 2000, except from 1993 to 1995. Since 1993, the Illinois State Water Survey has been monitoring the Lake Decatur watershed for trends in nitrate-N concentrations and loads and to identify any significant changes in the watershed. The purpose of the monitoring is to collect reliable hydrologic and water quality data throughout the watershed for use by city planners and resource managers to develop watershed management alternatives based on scientific data. This report presents the annual data for all seven years of monitoring (May 1993-April 2000) and monthly data for Year 7 of monitoring (May 1999-April 2000). Based on the seven years of data, it can be concluded that the unit of nitrate-N loads are relatively uniform over the entire watershed but tend to be slightly higher at the tributary streams in the upper Sangamon River watershed than at the Sangamon River stations closer to the lake. Nitrate-N loads vary with concentrations and streamflow and were the lowest in Year 7 because of the low streamflows during that year. Flow-weighted nitrate-N concentrations have been increasing during the study period at the Monticello station. The highest nitrate-N concentrations during the monitoring period were observed in years 6 and 7.
This report presents the climatology of Illinois tornadoes based on data from the 1916-1969 period, and offers a variety of general interest tornado facts. Illinois ranks eighth nationally in the number of tornadoes, but first in deaths and second in tornado damages. On the average, there are 10 tornadoes per year, occurring on five days. The annual average death rate from these storms is slightly over 19with an injured average of 110 people. A majority (65 percent) of Illinois tornadoes occur during March through June, with 15-21 April being the prime 7-day period. Over 40 percent occur between 1500 and 1800 CST, and 65 percent take place from 1400-2000 CST. Five of the outstanding Illinois tornado days of the 1916-1969 period are discussed in detail, including the famed Tri-State tornado of 18 March 1925, the most devastating tornado in the United States since systematic collection of tornado data began in 1916.The general information includes, among other items, basic definitions pertinent to tornadoes, safety precautions, formulation of a tornado forecast, and methods for remote detection of tornadoes.
The City of Decatur operates a series of ten groundwater wells in DeWitt and Piatt Counties that serve as an emergency water supply in times of low surface water levels in Lake Decatur. The City of Decatur contracted with Layne-Geosciences, Inc. (LGI) to develop a computer model of the groundwater system to simulate the effects of pumpage on the Mahomet Aquifer and surrounding wells. The LGI model was completed in April 1999. In response to lowering lake levels, Decatur began pumping their wells in November 1999 for 84 days at daily rates from 3 million gallons a day (mgd) to 16 mgd. The Illinois State Water Survey (ISWS) reviewed and tested the LGI model against the known drawdown encountered during the 84 days of operation. The LGI model was found to be only marginally successful in reproducing the measured water levels. The largest error occurred in the Piatt County area where the model significantly overpredicted the drawdown. These errors were the result of several factors, including errors in the aquifer thickness map, calibration to data only within 5 miles of the wellfield, errors in the location of pumping wells, the use of general head boundaries throughout the model, and, most importantly, the absence of a hydraulic connection between the Mahomet Aquifer, the Glasford Aquifer, and the Sangamon River near Allerton Park. Additional data available in the ISWS well records, and new data provided by Decatur through Guillou and Associates, Inc., indicate a connection between the aquifer system and the Sangamon River. Adding this connection represents a change in the conceptual model of the flow system not included in the LGI model. When this connection was added, a much closer match between observed and calculated water levels was obtained. Future work should focus on developing a more complete understanding of the connections between the aquifer system and the Sangamon River. Those efforts should include a pump test of the Cisco wellfield with complete monitoring of the river and aquifers. Monitoring of water levels at selected locations should continue and expand. The groundwater flow model should be re-calibrated using the new data and the improved understanding of the flow system. The results of these activities can provide an improved assessment of the potential of the Decatur wellfield for future use.
For more than ten years the State Water Survey has been making analyses of the various waters used by the people of the State. The work has been carried on in accordance with an Act of the Fortieth General Assembly entitled An Act to establish a chemical survey of the waters of the State of Illinois." This report contains data concerning the source of the water supplies and the quality of the water of cities which according to the census of 1900 had more than 1,000 inhabitants. The data concerning the supplies was obtained by correspondence with water works officials or city officers. Three series of letters were sent out and a direct-report, more or less complete, has been received from all but fourteen of the cities having more than 1,000 inhabitants. In a few instances data have been taken from previous works which treat of the municipal water supplies of Illinois. In nearly every case we have confirmed the data thus taken. There are also included analytical data which have been obtained by the survey since its foundation in 1897 to the end of 1906.
The purpose of this study was to use models to simulate the effect of the Stratton Dam operation, and possible structural modifications such as the addition of Foster gates, on flood stages and discharges in the Fox River and the Fox Chain of Lakes. The hydraulics and hydrology of Stratton Dam, the Fox River, and the Chain of Lakes were simulated for a wide range of historical flooding conditions and potential operation schemes. Responses for many different major flood conditions were analyzed, but two particular aspects of flood control were given special attention: 1) increasing outflow from the lakes in anticipation of a major flood, and 2) facilitating the flow release of the lakes by adding Foster gates at Stratton Dam and downstream at Algonquin Dam. This information will provide the IDOT-DWR with information for implementing possible modifications to the Stratton Dam operation during flood conditions.
A major study of Illinois droughts was conducted to develop a basis for quantitative assessments of drought conditions in Illinois. Such information should aid decision making related to local and state actions to ameliorate the effects of future droughts. Those who must deal directly with aspects of drought need to know 1) whether a drought is developing, 2) how severe the drought is at any given time, and 3) how long the drought will last. This report provides information to help address those questions. The report draws upon relevant results of in-depth studies of most critical aspects of drought, including the ways drought is reflected in the major components of the hydrologic cycle. Procedures are given for assessing drought presence and severity in terms of precipitation conditions, soil moisture conditions, shallow groundwater levels, and streamflows. Routine monitoring of these four conditions, coupled with the use of relationships developed in the drought study, serves to detect the onset of droughts in Illinois. Methods for determining the end of drought are not perfect, but the report discusses means by which the termination of drought can be estimated.
Sedimentation detracts from the use of any water supply lake by reducing lake depth and volume, with a reduction of reserve water supply capacity and possible burying of intake structures. Sedimentation of a reservoir is a natural process that can be accelerated or slowed by human activities in the watershed. Silver Lake is Located in Madison County, one mile northwest of Highland, Illinois. The location of the dame is 38 degrees 46' 00" north latitude and 89 degrees 42' 05" west longitude in Section 30, T.4N., R.5W., Madison County, Illinois. The dam impounds the East Fork of Silver Creek, a tributary of Silver Creek in the Kaskaskia River basin. The watershed is a portion of Hydrologic Unit 07140204 as defined by the U.S. Geological Survey. Construction of the lake was completed in 1962. The Silver Lake watershed consists of the 47.1-square-mile area drained by the East Fork of Silver Creek above the dam site. Land use in the watershed of the lake is mainly agricultural. Average annual precipitation in the area is 38.98 inches as measured at Greenville (1961-1990), and the average runoff (1912-1998) is approximately 10.0 inches (Shoal Creek near Breese). Average annual lake evaporation rates are 35.2 inches per year at St. Louis, Missouri. The Illinois State Water Survey conducted sedimentation surveys of Silver Lake in 1981 and 1984. In 1981, cross sections were laid out at 14 lines across the lake and surveyed. Sedimentation surveys of Silver Lake in 1984 and 1999 repeated as closely as possible the series of survey lines established during the 1981 survey. Sedimentation has reduced the capacity of Silver Lake from 7,322 acre-feet or ac-ft (2,386 million gallons) in 1962 to 5,832 ac-ft (1,900 million gallons) in 1999. Sediment accumulation rates in the lake have averaged 40.3 ac-ft per year from 1962-1999. Annual sedimentation rates for three separate periods, 1962-1981, 1981-1984, and 1984-1999, were 51.2, 63.0, and 21.9 ac-ft, respectively. Density analyses of the sediment samples indicate that sediment in the northern (upstream) portions of the lake has greater unit weight than sediment in the southern end of the lake. In general, coarser sediments are expected to be deposited in the upstream portion of a lake where the entrainment velocity of the stream is reduced to the much slower velocities of a lake environment. These coarser sediments tend to be denser when settled and are subject to shallow drying and higher compaction rates as a result of more frequent drawdown exposure in the shallow water environment. As the remaining sediment load of the stream is transported through the lake, increasingly finer particle sizes and decreasing unit weight are observed. The sedimentation rate for Highland Silver Lake is similar to the rates for other Illinois lakes of similar size and character. The sedimentation for Silver Lake is in the low to average ranged compared to other Illinois lakes.