The surface-water, ground-water, precipitation and biological data in this report are for the 1998 water year that began October 1, 1997, and ended September 30, 1998. Historical data are also provided in this report. Historical data consists of records stored on the Illinois District's computer system. For daily discharge records, this includes data for the period of record. The data available may include streamflow; stage and content for lakes and reservoirs; water quality for surface water; level and quality for ground water; precipitation and biological. The following sections of the introductory text are presented to provide users with a more detailed explanation of how the hydrologic data in this report were collected, analyzed, computed, and presented.
Each data station in this report, whether stream site or well, is assigned a unique identification number. This number is unique in that it applies specifically to a given station and to no other. The number usually is assigned when a station is first established and is retained for that station indefinitely. The systems used by the USGS to assign identification numbers for surface-water stations and for ground-water well sites differ, but both are based on geographic location. The "downstream order" system is used for regular surface-water stations, for rain gages and for biological sample sites at surface-water stations; the "latitude-longitude" system is used for wells and other sites not at surface-water stations.
Since October 1, 1950, the order of listing hydrologic-station records in USGS reports is in a downstream direction along the main stream. All stations on a tributary entering upstream from a mainstream station are listed before that station. A station on a tributary that enters between two mainstream stations is listed between them. Beginning water year 1998, stations on CD can be sorted in downstream direction or alphebetically by station name.
In assigning station numbers in downstream order, no distinction is made between partial-record stations and other stations; therefore, the station number for a partial-record station indicates downstream-order position in a list made up of both types of stations. Gaps are left in the series of numbers to allow for new stations that may be established; hence, the numbers are not consecutive. The complete eight-digit number for each station, such as 05527500, which appears just to the left of the station name, includes the two-digit part number "05" plus the six-digit downstream-order number "527500." The part number designates the major river basin; for example, part "05" is the upper Mississippi River Basin.
The identification numbers for wells and other sites not at surface-water stations are assigned according to the grid system of latitude and longitude (figure 2). The number consists of 15 digits. The first six digits denote the degrees, minutes, and seconds of latitude, the next seven digits denote degrees, minutes, and seconds of longitude, and the last two digits (assigned sequentially) identify the wells or other sites within a 1-second grid. This site-identification number, once assigned, is a pure number, and has no locational significance. In the rare instance where the initial determination of latitude and longitude is found to be in error, the station will retain its initial identification number; however, its true latitude and longitude will be listed in the LOCATION paragraph of the station description.
Records of stage and discharge may be complete or partial. Complete records of stage are obtained from stage-recording devices. Complete records of discharge are obtained based on discrete measurements and a stage-recording device. Data from these may be used to compute either instantaneous or mean-daily values for any instant or period of time within the period of record. Similarly, complete records of lake or reservoir content are those for which stage or content may be computed or estimated with reasonable accuracy for any instant or period of time. They may be obtained using a stage-recording device, but need not be. Stations with complete records are also referred to as continuous-record stations. Because daily mean stage, discharge or end-of-day content commonly are published for such stations, they are referred to as "daily stations." Locations of surface-water gaging stations for which data are given in this report are shown in figure 3 and figure 4 for stage records and figure 5 and figure 6 for discharge records. Data can be obtained on CD by selecting a location on a figure.
By contrast, partial records are obtained through discrete measurements, without the use of a stage-recording device, and pertain to only one or a few flow characteristics. Partial-record sites may include records, such as crest stage and miscellaneous measurements. Crest-stage records contain maximum stage and discharge for the water year and period of record. Miscellaneous measurements typically contain discharge measurements or measurements from special studies, such as low-flow seepage studies. Locations of crest-stage and miscellaneous discharge-measurement sites for which data are given in this report are shown in figure 7.
The data obtained at a complete-record gaging station on a stream or canal consist of a continuous record of stage, individual measurements of discharge throughout a range of stages, and notations regarding factors that may affect the relation between stage and discharge. These data, together with supplemental information, such as weather records, are used to compute daily discharges. The data obtained at a complete-record gaging station on a lake or reservoir consist of a record of stage and of notations regarding factors that may affect the relation between stage and lake content. These data are used with stage-area and stage-capacity curves or tables to compute water-surface areas and lake storage.
Continuous records of stage are obtained with digital recorders that punch stage values on paper tapes at selected time intervals, with electronic data loggers or satellite telemeters. Measurements of discharge are made with current meters and acoustical doppler flowmeters based on methods adapted by the USGS as a result of experience accumulated since 1880. These methods are described in standard textbooks, in Water-Supply Paper 2175 (Rantz and others, 1982), and in U.S. Geological Survey Techniques of Water-Resources Investigations (TWRI's), Book 3, Chapters A1 through A20 and Book 8, Chapters A2 and B2. The TWRI's are listed near the end of this introductory text. Methods are consistent with the American Society for Testing and Materials (ASTM) standards and generally follow the standards of the International Organization for Standards (IOS).
In computing discharge records, results of individual measurements are plotted against the corresponding stages, and stage-discharge relation curves are then developed. From these curves, rating tables indicating the approximate discharge for any stage within the range of the measurements are prepared. If it is necessary to define extremes of discharge outside the range of the current-meter measurements, the curves are extended using (1) logarithmic plotting; (2) velocity-area studies; (3) results of indirect measurements or peak discharge, such as slope-area or contracted-opening measurements, and computations of flow over dams or weirs; or (4) step-backwater techniques.
Daily mean discharges are computed by applying the daily mean stages (gage heights) to the stage-discharge curves or tables. If the stage-discharge relation is subject to change because of frequent or continual change in the physical features that form the control, the daily mean discharge is determined by the shifting-control method, in which correction factors based on the individual discharge measurements and notes of the personnel making the measurements are applied to the gage heights before the discharges are determined from the curves or tables. This shifting-control method also is used if the stage-discharge relation is changed temporarily because of aquatic growth or debris on the control. For some stations, formation of ice in the winter may so obscure the stage-discharge relations that daily mean discharges must be estimated from other information such as temperature and precipitation records, notes of observations, and records for other stations in the same or nearby basins for comparable periods.
At some surface-water gaging stations the stage-discharge relation is affected by the backwater from reservoirs, tributary streams, or other sources. This backwater effect necessitates the use of the slope method in which the slope or fall in a reach of the stream is a factor in computing discharge. The slope or fall is obtained by means of an auxiliary gage set at some distance from the base gage. At some stations the stage-discharge relation is affected by changing stage; at these stations the rate of change in stage is used as a factor in computing discharge.
At some gaging stations, acoustic velocity meter (AVM) systems are used to compute discharge. The AVM system measures the stream's velocity at one or more paths in the channel cross section. Coefficients are developed to relate this path velocity to the mean velocity in the channel cross section. Because the AVM sensors are fixed in position, the adjustment coefficients generally vary with stage. Cross-sectional area curves are developed to relate stage, recorded as noted above, to channel cross-sectional area. Discharge is computed by multiplying path velocity by the appropriate stage related coefficient and area.
In computing records of lake or reservoir content, it is necessary to have available from surveys, curves or tables that define the relation of stage and content. The application of stage to stage-content curves or tables gives the contents from which daily, monthly, or yearly changes then are determined. If the stage-content relation changes because of deposition of sediment in a lake or reservoir, periodic resurveys may be necessary to redefine the relation. Computations may become increasingly in error as time since the last survey elapses. Discharges over lake or reservoir spillways are computed from stage-discharge relation much as other stream discharges are computed.
For some gaging stations there are periods when no gage-height record is obtained, or the recorded gage height is so faulty that it cannot be used to compute daily discharge or content. This situation results when a recorder stops or otherwise malfunctions, when intakes become plugged, when a float freezes in a well, or for various other reasons. For such periods, the daily discharges are estimated from the recorded range in stage, previous or following record, discharge measurements, weather records, and comparison with other station records from the same or nearby basins. Likewise, daily content may be estimated from operator's logs, previous or following record, inflow-outflow studies, and other information. Information explaining how estimated daily-discharge values are identified in station records is included in the next two sections, "Data Presentation" (REMARKS paragraph) and "Identifying Estimated Daily Discharge."
Streamflow data in this report are presented in a format different from the format in data reports prior to the 1991 water year. The major changes are that statistical characteristics of discharge now appear in tabular summaries following the traditional data table and less information is provided in the text or station manuscript above the table. These changes represent the results of a pilot program to reformat the annual water-data report to meet current user needs and data preferences. Beginning with water year 1998 and the presentation of data on CD, mean-daily stage and discharge data for the current year are published in tabular data format (RDB format). In addition, historical stage and discharge data, when available from the Illinois District's computer system, are provided in tabular data format on CD. These RDB formatted data are provided in addition to the traditional data tables.
The records published for each continuous-record surface-water discharge station (gaging station) after 1991 consist of four parts: the manuscript or station description; the data table of daily mean values of discharge for the current water year with summary data; a tabular statistical summary of monthly mean flow data for a designated period, by water year; and a summary statistics table that includes statistical data of annual, daily, and instantaneous flows as well as data pertaining to annual runoff, 7-day low-flow minimums, and flow duration.
GAGE.--The type of gage in current use, the datum of the current gage referred to sea level (see Definition of Terms), and a condensed history of the types, locations, and datums of previous gages are given under this heading.
REMARKS.--This paragraph is used to present information relative to the accuracy of the records, to special methods of computation, and to conditions that affect natural flow at the station. In addition, information may be presented, such as whether the site has telemetry. For reservoir stations, information is given on the dam forming the reservoir, the capacity, outlet works and spillway, and purpose and use of the reservoir.
COOPERATION.--Records provided by a cooperating organization or obtained for the USGS by a cooperating organization are identified here.
EXTREMES FOR PERIOD OF RECORD.--Extremes may include maximum and minimum stages and maximum and minimum discharges or content. Unless otherwise qualified, the maximum discharge or content is the instantaneous maximum corresponding to the highest stage that occurred. The highest stage may have been obtained from a recorder, a crest-stage gage, or by direct observation of a nonrecording gage. If the maximum stage did not occur on the same day as the maximum discharge or content, it is given separately. Similarly, the minimum is the instantaneous minimum discharge, unless otherwise qualified, and was determined and is reported in the same manner as the maximum.
EXTREMES OUTSIDE PERIOD OF RECORD.--Included here is information concerning major floods or unusually low flows that occurred outside the stated period of record. The information may or may not have been obtained by the USGS.
EXTREMES FOR CURRENT YEAR.--Extremes given here are similar to those for the period of record, except the peak discharge listing may include secondary peaks. For stations meeting certain criteria, all peak discharges and stages that occurred during the water year and were greater than a selected base discharge are presented under this heading. Peaks greater than the base discharge, excluding the highest, are referred to as secondary peaks. Peak discharges are not published for canals, ditches, drains, or streams for which the peaks are subject to substantial control by man. The time of occurrence for peaks is expressed in 24-hour central standard time. For example, 12:30 a.m. is 0030, and 1:30 p.m. is 1330. The minimum for the current water year appears below the table of peak data. Beginning water year 1998, "Extremes for current year" and secondary peaks have been eliminated. These extremes, however, are provided in the summary statistics table with the exception of the peaks greater than the base discharge.
REVISIONS.--If a critical error in published records is discovered, a revision is included in the first report published following discovery of the error.
Although rare, occasionally the records of a discontinued gaging station may need revision. Because, for these stations, there would be no current or, possibly, future station manuscript published to document the revision in a "Revised Records" entry, users of data for these stations, who obtained the record from previously published data reports may wish to contact the District Office (address given in the insert of the CD) to determine if the published records were ever revised after the station was discontinued. Of course, if the data for a discontinued station were obtained by computer retrieval, the data would be current and there would be no need to check because any published revision of data is always accompanied by revision of the corresponding data in computer storage.
Manuscript information for lake or reservoir stations differs from that for stream stations in the nature of the "Remarks" and in the inclusion of a skeleton stage-capacity table when daily contents are given.
Headings for AVERAGE DISCHARGE and EXTREMES FOR PERIOD OF RECORD have been deleted for most stations and the information contained in these paragraphs is now presented in the tabular summaries following the discharge table. The EXTREMES FOR PERIOD OF RECORD paragraph was retained for those stations where the period of record differed from the period used for the tabular summary statistics. No changes have been made to the data presentations of lake contents.
Data table of daily mean values
For stage-only stations, the daily tables contain mean gage height for those days that have record for the whole day. Those days without a complete day of record are noted with dashes (--). The monthly summary of the table contains; "MEAN", which gives the average daily stage in feet for each month; "MAX" and "MIN", which give the maximum and minimum daily stages for each month. If a complete year of mean-daily stages is available, then the "MEAN", "MAX", and "MIN" for the calendar year and/or water year are provided at the bottom of the table.
Statistics of monthly mean data
A tabular summary of the mean (line headed "MEAN"), maximum (line headed "MAX"), and minimum (line headed "MIN") of monthly mean flows for each month for a designated period is provided below the annual monthly summary for continuous-record discharge stations. The water years of the first occurrence of the maximum and minimum monthly flows are provided immediately below those figures. The designated period will be expressed as "FOR WATER YEARS - , BY WATER YEAR (WY)," and will list the first and last water years of the range of years selected from the PERIOD OF RECORD paragraph in the station manuscript. It will consist of the entire station record within the specified water years, inclusive, including complete months of record for partial water years, if any, and may coincide with the period of record for the station. The water years for which the statistics are computed will be consecutive, unless a break in the station record is indicated in the manuscript.
HIGHEST ANNUAL MEAN.--The maximum annual mean discharge occurring for the designated period.
LOWEST ANNUAL MEAN.--The minimum annual mean discharge occurring for the designated period.
HIGHEST DAILY MEAN.--The maximum daily mean discharge for the year or for the designated period.
LOWEST DAILY MEAN.--The minimum daily mean discharge for the year or for the designated period.
90 PERCENT EXCEEDS.--The discharge that has been exceeded 90 percent of the time for the designated period.
Data for partial-record discharge sites are presented in two tables if available. The first is a table of annual maximum stage and discharge at crest-stage sites, and the second is a table of miscellaneous discharge measurements. Miscellaneous measurements are generally made in times of drought or flood to give better areal coverage to those events.
Estimated daily-discharge values published in water-discharge tables are identified by flagging individual daily values with the letter symbol "e" and a footnote.
The accuracy of streamflow records depends primarily on (1) the stability of the stage-discharge relation or, if the control is unstable, the frequency of discharge measurements; and (2) the accuracy of measurements of stage, measurements of discharge, and interpretation of records.
The accuracy attributed to the records is indicated under "REMARKS." "Excellent" means that about 95 percent of the daily discharges are within 5 percent of the true value; "good," within 10 percent; and "fair," within 15 percent. Records that do not meet the criteria mentioned are rated "poor." Different accuracies may be attributed to different parts of a given record.
Daily mean discharges in this report are given to the nearest hundredth of a cubic foot per second for values less than 1 ft3/s; to the nearest tenth between 1.0 and 10 ft3/s; to whole numbers between 10 and 1,000 ft3/s; and to 3 significant figures for more than 1,000 ft3/s. The number of significant figures used is based solely on the magnitude of the discharge value. The same rounding rules also apply to discharges listed for partial-record stations.
Discharge at many stations may not reflect natural runoff because of the effects of diversion, consumption, regulation by storage, increase or decrease in evaporation due to artificial causes, or other factors. For such stations, values of cubic feet per second per square mile and of runoff, in inches, are not published unless satisfactory adjustments can be made for diversions, for changes in contents of reservoirs, or for other changes incident to use and control. Evaporation from a reservoir is not included in the adjustments for changes in reservoir contents, unless it is so stated. Even at those stations where adjustments are made, large errors in computed runoff may occur if adjustments or losses are large in comparison with the observed discharge.
Information used in the preparation of the records in this publication, such as discharge-measurement notes, instantaneous gage-height and discharge records, temperature measurements, and rating tables are available from the USGS Illinois District office whose address is given in the insert of the CD. Information on the availability of the unpublished information may be obtained from the USGS Illinois District office.
Records of surface-water quality typically include chemical, microbiological and sediment data. These records ordinarily are obtained at or near surface-water gaging stations because interpretation of records of surface-water quality nearly always requires corresponding discharge data. Records of surface-water quality in this report may involve a variety of types of data and measurement frequencies.
Water-quality data for surface-water sites are grouped into one of three classifications. A continuing-record station is a site where data are collected on a regularly scheduled basis. Frequency may be one or more times daily, weekly, monthly, or quarterly. A partial-record station is a site where limited water-quality data are collected systematically over a period of years. Frequency of sampling is usually less than quarterly. A miscellaneous sampling site is a location other than a continuing- or partial-record station, where random samples are collected to give better areal or temporal coverage to define water-quality conditions in a river basin. Locations of stations for which surface-water quality data appear in this report are shown in figure 8.
In obtaining surface-water-quality data, a major concern is that the data obtained represent the ambient quality of the water. To assure this, certain measurements, such as water temperature, pH, and dissolved oxygen concentration, need to be made on-site when the samples are taken. To assure that measurements made in the laboratory represent field conditions, carefully prescribed procedures need to be followed in collecting the samples, in treating the samples to prevent changes in quality pending analysis, and in shipping the samples to the laboratory. Procedures for on-site measurements and for collecting, treating, and shipping samples are detailed in TWRI Book 1, Chapter D2; Book 3, Chapter C2; Book 5, Chapters A1, A3, and A4; Shelton (1994); Horowitz and others (1994); and other USGS publications. These methods are consistent with ASTM standards and generally follow IOS standards. Detailed information on collecting, treating, and shipping samples also may be obtained from the USGS Illinois District office.
One sample can adequately define the water quality at a given time if the mixture of constituents throughout the stream cross section is homogeneous. However, the concentrations of constituents at different locations in the cross section may vary widely with different rates of water discharge, depending on the source of material and the turbulence and mixing of the stream. Some streams must be sampled through several vertical sections to obtain a representative sample. Whether samples are obtained from the centroid of flow or from several verticals depends on flow conditions and other factors that must be evaluated by the collector.
Chemical-quality data published in this report are considered to be the most representative values available for the stations listed. The values reported represent water-quality conditions at the time of sampling and were analyzed on the basis of current sampling techniques and methods of analysis used by the National Water Quality Laboratory.
For chemical-quality stations equipped with digital monitors, the records consist of daily maximum, minimum, and mean values for each constituent measured and are based on hourly values.
Water temperatures are measured at all of the water-quality stations. In addition, water temperatures are taken at the time of discharge measurements for discharge stations. Large streams have small diel temperature changes; shallow streams may have a daily range of several degrees and may follow closely the changes in air temperature. Some streams may be affected by waste-heat discharges.
At stations where recording instruments are used, either mean temperatures or maximum and minimum temperatures for each day are published. Water temperatures measured at the time of discharge measurements are available from the USGS Illinois District office.
At daily sediment stations, suspended-sediment concentrations are determined from samples collected by using depth-integrating samplers. Samples usually are obtained at several verticals in the cross section, and/or a single sample may be obtained at a fixed point and a coefficient applied to compute the mean concentration in the cross section. Data tables present daily water discharge, suspended-sediment concentration and suspended-sediment discharge.
During periods of rapidly changing flow or rapidly changing concentration, samples may have been collected more frequently (twice daily or, in some instances, hourly). The published sediment discharges for days of rapidly changing flow or concentration were computed by the subdivided-day method (time-discharge weighted average). Therefore, for those days when the published sediment discharge value differs from the value computed as the product of discharge times mean concentration times 0.0027, the reader can assume that the sediment discharge for that day was computed by the subdivided-day method. For periods when no samples were collected, daily discharges of suspended sediment were estimated on the basis of water discharge, sediment concentrations observed immediately before and after the periods, and suspended-sediment discharges for other periods of similar water discharge. Methods used in the computation of sediment records are described in TWRI book 3, chapters C1 and C3. These methods are consistent with ASTM standards and generally follow IOS standards. Locations of stations for which records of suspended sediment appear in this report are shown in figure 8.
At other stations, suspended-sediment samples were periodically collected by depth integrated sampling at several verticals in the stream cross section. Although data collected periodically may represent conditions only at the time of observations, such data are useful in establishing seasonal relations between quality and streamflow in determining long-term sediment-discharge characteristics of the stream.
In addition to the records of suspended-sediment discharge, records of the periodic measurements of the particle-size distribution of the suspended sediment and bed material may be included for some stations.
Sediment samples are analyzed at the USGS sediment laboratory in Louisville, Kentucky. Samples for indicator bacteria are analyzed locally. All other samples are analyzed in the USGS National Laboratory in Denver, Colorado, or the Illinois Environmental Protection Agency laboratory in Champaign, Illinois. Methods used in analyzing sediment samples and in computing sediment records are given in TWRI book 5, chapter C1. Methods used by the USGS laboratories are given in TWRI book 1, chapter D2; book 3, chapter C2; book 5, chapters A1, A3, A4, and A5; and other USGS publications. These methods are consistent with ASTM standards and generally follow IOS standards. The following codes identify the agency or office having the principal responsibility for collecting and (or) analyzing water samples.
| Agency | Agency Code |
|---|---|
| U.S. Geological Survey (general) | 1028 |
| Illinois Environmental Protection Agency | 17002 |
| U.S. Geological Survey, Denver Laboratory | 80020 |
| U.S. Geological Survey, Illinois District | 81700 |
For continuing-record stations, information pertinent to the history of station operation is provided in descriptive headings preceding the tabular data. These descriptive headings give details regarding location, drainage area, period of record, type of data available, instrumentation, general remarks, cooperation, and extremes for parameters currently measured daily.
The following information, as appropriate, is provided with each continuous-record station. Comments that follow clarify information presented under the various headings of the station description.
LOCATION.--See Data Presentation under "Records of Stage and Discharge;" same comments apply.
DRAINAGE AREA.--See Data Presentation under "Records of Stage and Discharge;" same comments apply.
PERIOD OF RECORD.--This indicates the periods for which there are published water-quality records for the station. The periods are shown separately for records of parameters measured daily or continuously and those measured less than daily. For those measured daily or continuously, periods of record are given for the parameters individually.
INSTRUMENTATION.--Information on instrumentation is given only if a water-quality monitor temperature recorder, pumping sediment sampler, or other sampling device is in operation at a station.
REMARKS.--Remarks provide added information pertinent to the collection, analysis, or computation of the records.
COOPERATION.--Records provided by a cooperating organization or obtained for the USGS by a cooperating organization are identified here.
EXTREMES.--Maximums and minimums are given only for parameters measured daily or more frequently. None are given for parameters measured weekly or less frequently, because the true maximums or minimums may not have been sampled. Extremes, when given, are provided for both the period of record and for the current water year.
REVISIONS.--If errors in published surface-water-quality records are discovered after publication, appropriate updates are made to the Water-Quality File in the Illinois District's computer system. The revisions to the database are not published.
The surface-water-quality records for partial-record stations and miscellaneous sampling sites are published in separate tables in the surface-water quality section titled "Miscellaneous". No descriptive statements are given for these records. Each station is published with its own station number and name in the regular downstream-order sequence.
Measurements of ground-water levels are made under varying conditions, but the methods of measurement are standardized to the extent possible. The equipment and measuring techniques used at each observation well ensure that measurements at each well are of consistent accuracy and reliability. Locations of wells for which water-level data are given in this report are shown in figure 9.
Tables of ground-water-level data are presented by counties arranged in alphabetical order. The prime identification number for a given well is the 15-digit number that appears in the upper left corner of the table. The secondary identification number is the local well number, an alphanumeric identifier, derived from the township-range location of the well.
Ground-water-level records are obtained by direct measurements using a steel tape or from an electronic water-stage recorder. The water-level measurements in this report are given in feet with reference to land-surface datum (lsd). Land-surface datum is a datum plane that is approximately at land surface at each well. If known, the elevation of the land-surface datum is given in the well description. The height of the measuring point (MP) above or below land-surface datum is given in each well description. The method and frequency of measurement are given in the station description.
Ground-water levels are reported to as many significant figures as can be justified by the local conditions. For example, in a measurement of a depth to water of several hundred feet, the error in determining the absolute value of the total depth to water may be a few tenths of a foot, whereas the error in determining the net change of water level between successive measurements may be only a hundredth or a few hundredths of a foot. For lesser depths to water, the accuracy is greater. Accordingly, most measurements are reported to a hundredth of a foot, but some are given to a tenth of a foot or a larger unit.
Each well record consists of two parts: the station description and the data table of water levels observed during the current water year. The description of the well is presented first through the use of descriptive headings preceding the tabular data. The comments to follow clarify information presented under the various headings of the well description.
LOCATION.--This paragraph follows the well-identification number and reports the latitude and longitude (given in degrees, minutes, and seconds); a landline location designation; the hydrologic-unit number; the distance and direction from a geographic point of reference; and the owner's name.
AQUIFER.--This entry designates by name (if a name exists) and geologic age of the aquifer(s) open to the well.
WELL CHARACTERISTICS.--This entry describes the well in terms of depth, diameter, casing depth and (or) screened interval, method of construction, use, and additional information such as casing breaks, collapsed screen, and other changes since construction.
INSTRUMENTATION.--This paragraph provides information on both the frequency of measurement and the collection method used, allowing the user to better evaluate the reported water-level extremes by knowing whether they are based on weekly, monthly, or some other frequency of measurement.
DATUM.--This entry describes both the measuring point and the land-surface elevation at the well. The measuring point is described physically (such as top of collar, notch in top of casing, plug in pump base and so on), and in relation to land surface (such as 1.3 ft above land-surface datum). The elevation of the land-surface datum is described in feet above (or below) sea level; it is reported with a precision depending on the method of determination.
REMARKS.--This entry describes factors that may affect the water level in a well or the measurement of the water level. It should identify wells that also are water-quality observation wells, and may be used to acknowledge the assistance of local (non-USGS) observers.
PERIOD OF RECORD.--This entry indicates the period for which there are published records for the well. It reports the month and year of the start of publication of water-level records by the USGS and the words "to current year" if the records are to be continued into the following year. Periods for which water-level records are available, but are not published by the USGS, may be noted.
EXTREMES FOR PERIOD OF RECORD.--This entry contains the highest and lowest water levels of the period of published record, with respect to land-surface datum, and the dates of their occurrence.
A table of water levels follows the station description for each well. Water levels are reported in feet below land-surface datum and all taped measurements of water level are listed. For wells equipped with recorders, only abbreviated tables are published; generally, only water-level lows are listed for every fifth day and at the end of the month (eom). The highest and lowest water levels of the water year and their dates of occurrence are shown on a line below the abbreviated table. Because all values are not published for wells with recorders, the extremes may be values that are not listed in the table. Missing records are indicated by dashes in place of the water level.
Records of ground-water quality in this report differ from other types of records in that, for most sampling sites, they consist of only one set of measurements for the water year. The quality of ground water ordinarily changes only slowly; therefore, samples for water-quality analyses are taken from wells on an infrequent basis--about once every 3 to 4 years. Typically, 3 to 4 wells are sampled each year. Frequent measurement of the same constituents is not necessary unless one is concerned with a particular problem, such as monitoring for trends in nitrate concentration. In the special cases where the quality of ground water may change more rapidly, more frequent measurements are made to identify the nature of the changes. Locations of wells for which water-quality data are given in this report are shown in figure 9.
Most methods for collecting and analyzing water samples are described in the USGS TWRI publications referred to in the "On-Site Measurements and Sample Collection" and the "Laboratory Measurements" sections in this data report. In addition, the TWRI Book 1, Chapter D2, describes guidelines for the collection and field analysis of ground-water samples for selected unstable constituents. The values reported in this report represent water-quality conditions at the time of sampling as much as possible, consistent with available sampling techniques and methods of analysis. These methods are consistent with ASTM standards and generally follow IOS standards. All samples were obtained by trained personnel. The wells sampled were pumped long enough to assure that the water collected came directly from the aquifer and had not stood for a long time in the well casing where it would have been exposed to the atmosphere and to the material, possibly metal, compose the casings.
The records of ground-water quality are listed alphabetically by county, and are identified by well number. The prime identification number for wells sampled is the 15-digit number derived from the latitude-longitude locations. No descriptive statements are given for ground-water-quality records; however, the well number, depth of well, date of sampling, and other pertinent data are given in the table containing the chemical analyses of the ground water. The "Remark" codes listed near the end of this introductory text also are applicable to ground-water-quality records.
Rainfall values were determined using tipping-bucket rain gages. Rainfall data were collected by electronic data loggers in 0.01-in. increments every 5 minutes. Twenty-four hour rainfall totals are tabulated and presented. A 24-hour period extends from just past midnight the previous day to midnight the current day. Snowfall-affected data can result during cold weather when snow fills the rain-gage funnel and then melts as temperatures rise. Snowfall-affected data are subject to appreciable errors. Snowfall-affected readings were determined by routine field trips and by comparing recorded data with snowfall, snow depth, and temperature and precipitation data from the National Oceanic and Atmospheric Administration precipitation gages. Daily values for periods of missing record were estimated using amounts recorded at nearby rain gages and the reciprocal-distance-squared equation (U.S. Department of Commerce, 1972, p. 3-11). Locations of rain gages for which data are given in this report are shown in figure 10.
Precipitation records collected at surface-water gaging stations are identified by the same station number and name as the gaging station. Where a surface-water, daily-record station is not available, the precipitation record is published with its own name and latitude-longitude identification number.
Information pertinent to the history of a precipitation station is provided in descriptive headings preceding the tabular data. These descriptive headings give details regarding location, period of record, record accuracy, general remarks, and maximum daily values.
The following information, as appropriate, is provided with each precipitation station. Comments that follow clarify information presented under the various headings of the station description.
LOCATION.--Information on locations is obtained from the most accurate maps available. A consideration used in the determination of site locations was the proximity of existing structures and minimal obstructions.
PERIOD OF RECORD.--See Presentation section under "Stage and Discharge;" same comments apply.
ACCURACY.--Information about the physical characteristics of the gage that may effect accuracy of the recorded values.
REMARKS.--Remarks provide added information pertinent to the collection, analysis, or computation of records.
MAXIMUM FOR PERIOD OF RECORD.--Maximum daily precipitation value for the period of record.
MAXIMUM FOR CURRENT YEAR.--Maximum daily precipitation value for the current year.
Stream biological-community data are obtained by the National Water-Quality Assessment (NAWQA) program. The NAWQA program collects fish, macroinvertebrate, and algae community data. Locations of biological sample sites for which data are given in this report are shown in figure 11.
Biological samples are collected according to protocols established by the NAWQA program (Cuffney and others, 1993; Meador and others, 1993; Porter and others, 1993). Macroinvertebrate and algae sampling typically include both quantitative and qualitative samples. Fish, macroinvertebrate, and algae samples are collected within defined reaches, or lengths of stream, which are at or near surface-water quality stations. When multiple samples are collected from a station, they are collected from separate reaches. Letter designations are given to these reaches. For more information on the location of specific reaches, contact the Illinois District office at the address given in the CD insert or by telephone (217-344-0037).
The period and method of sample processing depends on the type of organism sampled. Fish are primarily identified in the field with selected specimens collected for identification, verification, or vouchering at the Illinois Natural History Survey, Champaign. Macroinvertebrate and algae samples are sent to the USGS National Water-Quality Laboratory for processing, enumeration and taxonomic identification.
Data tables are arranged by taxonomic group and water year of sample. In this report, fish and macroinvertebrate data are presented from a
study of the lower Illinois River Basin.