Each data station in this report, whether stream site, well, or rain gage, 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 other data sites differ, but both are based on geographic location. The "downstream order" system is used for surface-water stations; the "latitude-longitude" system is used for wells and miscellaneous data sites.

Since October 1, 1950, surface-water records in USGS reports have been listed in order of downstream direction along the main-stream. All stations on a tributary entering upstream from a main-stream station are listed before that station. A station on a tributary that enters between two main-stream stations is listed between them. Beginning in water year 1998, stations listed in this report can be sorted in downstream direction or alphabetically 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. An eight-digit number for a station, such as 05527500, which appears on station lists in this report 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.  In areas of high station density, more than eight digits may be used for a station number.

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 discharge and stage may be complete or partial.  Complete records of discharge are obtained based on discrete measurements and a stage-recording device. Complete records of stage are obtained from stage-recording devices.  Data from these devices may be used to compute either instantaneous or mean-daily values for any instant or period of time within the period of record. Stations with complete records also are referred to as continuous-record stations.  Locations of surface-water gaging stations  for which water year 2004 discharge are given in this report are shown in figure 3. Gaging stations for which stage data for water year 2004 are given in this report are shown in figure 4. Data can be accessed by selecting a location on a figure.

As streamflow information needs far exceeds the number of gaging stations feasible to operate at one time, the USGS collects limited streamflow data at sites other than continuous-record gaging stations. When limited streamflow data are collected on a systematic basis over a period of years for use in hydrologic and hydraulic analyses, the site at which the data are collected is called a partial-record station. Data collected at these partial-record stations are usable in low-flow or floodflow analyses, depending on the type of data collected. Partial-record sites may include records, such as crest stage and miscellaneous measurements. Crest-stage records contain maximum stage and discharge by water year and for the period of record. Miscellaneous measurements typically contain discharge measurements or measurements from special studies, such as low-flow seepage studies.  Crest-stage gage sites for water year 2004 are shown in figure 4, whereas miscellaneous-measurement sites are shown in figure 3.

The data obtained at a continuous-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 discharges. The data obtained at a continuous-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 were obtained with electronic data loggers or satellite telemeters. Measurements of discharge are made with current meters and acoustical 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, which may be accessed at http://water.usgs.gov/pubs/twri/.  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 with the corresponding stages, and stage-discharge relation curves are then developed. From these curves, rating tables are prepared that indicate the approximate discharge for any stage within the range of the measurements.  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) and gage values of discharge are computed by applying the 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, 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 obscure the stage-discharge relations such 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. Gage values of discharge computed from recorded gage heights are not available for days when daily mean discharge was estimated because of ice formation.

At some surface-water gaging stations, the stage-discharge relation is affected by 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 curves, or tables available from surveys, 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 lake or reservoir 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 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, gage values of gage height and gage values of discharge are not available, and daily discharges are estimated from the recorded range in stage, previous or subsequent record, field notes, 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 subsequent record, inflow-outflow studies, and other information. Information explaining how estimated daily-discharge values are identified in station records is included in the section titled "Identifying Estimated Daily Discharge."

Daily-value streamflow tables in this report are presented in a format different from the format in data reports prior to the 1991 water year. The major changes since 1991 are that statistical characteristics of discharge appear in tabular summaries following the traditional annual data 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.

Daily-value tables for each continuous-record surface-water discharge station (gaging station) after 1991 consist of four parts: the station description; the data table of daily mean values of discharge for a 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.

Beginning with water year 1998 and the presentation of data in an electronic report, mean-daily discharge and stage data are published in annual daily-value tables. In addition, daily-value data are provided in text files when approved and available from the USGS Illinois Water Science Center's computer system.

Station description

The description provides, under various headings, information such as station location; period of record; extremes for period of record; record accuracy; and other remarks pertinent to the station. An annual daily-value table includes a station description (listed first) followed by a daily-value table. Station descriptions also are provided separate from annual data tables in the electronic report.

The following surface-water gaging information, as appropriate, is provided in station descriptions: 

LOCATION.--Reports the latitude and longitude (generally in degrees, minutes, and seconds) with the associated national horizontal datum in parentheses. Information on locations is obtained from the most accurate maps available. The latitude and longitude of a station is determined from maps and/or global positioning systems. The location of the data station with respect to the cultural and physical features in the vicinity and with respect to the reference place mentioned in the station name is given. River mileages were determined by methods given in "River Mileage Measurement," Bulletin 14, Revision of October 1968, prepared by the Water Resources Council, and in "River Mileages and Drainage Areas for Illinois Streams" (Healy, 1979a, 1979b).

DRAINAGE AREA.--Drainage areas are measured using the most accurate maps available. Because the type of maps available varies from one drainage basin to another, the accuracy of drainage areas likewise varies. Drainage areas are updated as better maps become available.

PERIOD OF RECORD.--Indicates the period for which records have been published for the station or for an equivalent station. An equivalent station is one that was in operation at a time that the present station was not, and whose location was such that flow at it can be considered reasonably equivalent to flow at the present station. Starting water year 2000, the period of record contains a list of the various data types published with their corresponding period(s) of record.

REVISED RECORDS.--Because of new information, published records occasionally are found to be incorrect, and revisions are given in later reports. Listed under this heading are all the reports in which revisions have been published for the station and the water years to which the revisions apply. If a revision did not include daily, monthly, or annual figures of discharge, that fact is noted after the year as follows: "(M)" means that only the instantaneous maximum discharge was revised; "(m)" that only the instantaneous minimum was revised; and "(P)" that only peak discharges were revised. If the drainage area has been revised, the report in which the most recently revised figure was first published is given. 

GAGE.--The type of gage equipment in current use, the datum of the current gage referred to the associated national vertical datum (see Definition of Terms), and a condensed history of the types, locations, and datums of previous gages are given under this heading.

REMARKS.--Used to present long-term special methods of computation and conditions that affect natural flow at the station. 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.

REMARKS FOR CURRENT YEAR.--Established water year 2000 and used to present information relative to the accuracy of the data being presented, to special methods of computation, and to conditions that affect natural flow at the station during the current water year.

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, a high-water mark, 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 maximums.

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 "Extremes for Period of Record", except the peak discharge listing may include secondary peaks. Prior to water year 1998 and 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. Secondary peak discharges are not published for canals, ditches, drains, or streams for which the peaks are subject to substantial control by man.  Beginning water year 1998, "Extremes for current year" and peaks greater than the base discharge were eliminated for discharge stations. These extremes, however, are provided in the summary statistics table with the exception of the peaks greater than the base discharge which are provided in the "Peak Flow" data section.  The "Extremes for current year", however, are provided for stage-only and precipitation stations.

REVISIONS.--Records are revised if errors in published records are discovered.  Appropriate updates are made in the USGS distributed data system, NWIS,and subsequently to its Web-based national data system, NWISWeb (http://water.usgs.gov/nwis/nwis).  Users are encouraged to obtain all required data from NWIS or NWISWeb to ensure that they have the most recent data updates.  Updates to NWISWeb are made on an annual basis.

Although rare, occasionally the records of a discontinued gaging station may need revision. Because no current or, possibly, future station description would be 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 USGS Illinois Water Science Center  (address given in the section titled "Access to USGS Water Data") to determine if the published records were revised after the station was discontinued. However, if the data for a discontinued station were obtained by computer retrieval, the data would be current.  Any published revision of data always is accompanied by revision of the corresponding data in computer storage.

Station description information for lake or reservoir stations differs from that for stream stations in the nature of the "Remarks" and in the inclusion of a stage-capacity table when daily volumes are given.

Headings for AVERAGE DISCHARGE have been deleted for most stations and the information contained in this paragraph is now presented in the tabular summaries following the discharge table. No changes have been made to the data presentations of lake contents.

Daily mean values

The daily table of discharge records for surface-water gaging stations gives mean discharge for each day of the water year. In the monthly summary for the table, the line headed "TOTAL" gives the sum of the daily figures for each month; the line headed "MEAN" gives the average flow in cubic feet per second for the month; and the lines headed "MAX" and "MIN" give the maximum and minimum daily mean discharges, respectively, for each month. Usually, discharge for the month also is expressed in cubic feet per second per square mile (line headed "CFSM"); or in inches (line headed "IN."); or in acre-feet (line headed "AC-FT"). Figures for cubic feet per second per square mile and runoff in inches or in acre-feet may be omitted if there is extensive regulation or diversion or if the drainage area includes large noncontributing areas. At some stations, monthly and (or) yearly observed discharges are adjusted for reservoir storage or diversion, or diversion data or reservoir contents are given. These figures are identified by a symbol and corresponding footnote.

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, and "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 description. The period of record will consist of the station record for the specified water years including complete months of record for partial water years, if any, and may or may not coincide with the entire 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 station description.

Summary statistics

A table titled "SUMMARY STATISTICS" follows the statistics of monthly mean data tabulation for continuous-record discharge stations. This table consists of four columns, with the first column containing the line headings of the statistics being reported. The table provides a statistical summary of yearly, daily, and instantaneous flows, not only for the current water year but also for the previous calendar year and for a designated period, as appropriate. The designated period selected, "WATER YEARS     -   ," will consist of the station record for the specified water years, including complete months of record for partial water years, if any, and may or may not coincide with the entire 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 station description. All of the calculations for the statistical characteristics designated ANNUAL (see line headings below), except for the "ANNUAL 7-DAY MINIMUM" statistic, are calculated for the designated period based on complete water years. The other statistical characteristics may be calculated using partial water years.

The date or water year, as appropriate, of the first occurrence of each statistic reporting extreme values of discharge is provided adjacent to the statistic. Repeated occurrences are noted in footnotes. Because the designated period may not be the same as the station period of record published in the description, occasionally the dates of occurrence listed for the daily and instantaneous extremes in the designated-period column may not be within the selected water years listed in the heading. When this situation occurs, it will be noted in the REMARKS paragraph or in footnotes. Selected streamflow-duration curve statistics and runoff data also are given. Runoff data may be omitted if there is extensive regulation or diversion of flow in the drainage basin.

The following summary statistics, as appropriate, are provided with each continuous-record discharge station:

ANNUAL TOTAL.--The sum of the daily mean values of discharge for the year. At some stations, the annual total discharge is adjusted for reservoir storage or diversion. The adjusted figures are identified by a symbol and corresponding footnotes.

ANNUAL MEAN.--The arithmetic mean of the individual daily mean discharges for the year noted or for the designated period. At some stations, the yearly mean discharge is adjusted for reservoir storage or diversion. The adjusted figures are identified by a symbol and corresponding footnotes.

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.

ANNUAL 7-DAY MINIMUM.--The lowest mean discharge for 7 consecutive days for a calendar year or a water year. Note that most low-flow frequency analyses of annual 7-day minimum flows use a climatic year (April 1 through March 31). The date shown in the summary statistics table is the initial date of the 7-day period. (This value should not be confused with the 7-day, 10-year low-flow statistic.)

MAXIMUM PEAK FLOW.--The maximum instantaneous peak discharge occurring for the water year or designated period. Occasionally, the maximum flow for a year may occur at midnight at the beginning or end of the year, on a recession from or rise toward a higher peak in the adjoining year. In this case, the maximum peak flow is given in the table and the maximum flow may be reported in a footnote.

MAXIMUM PEAK STAGE.--The maximum instantaneous peak stage occurring for the water year or designated period. Occasionally, the maximum stage for a year may occur at midnight at the beginning or end of the year, on a recession from or rise toward a higher peak in the adjoining year. In this case, the maximum peak stage is given in the table and the maximum stage may be reported in a footnote. If the dates of occurrence of the maximum peak stage and maximum peak flow are different, a footnote may be used to provide further information.

INSTANTANEOUS LOW FLOW.--The minimum instantaneous discharge occurring for the water year or for the designated period.

ANNUAL RUNOFF.--Indicates the total quantity of water in runoff for a drainage area for the year. Data tables may use any of the following units of measurement in presenting annual runoff data:

Acre-foot (AC-FT) is the quantity of water required to cover 1 acre to a depth of 1 foot and is equivalent to 43,560 cubic feet or about 326,000 gallons or 1,233 cubic meters.

Cubic feet per second per square mile (CFSM) is the average number of cubic feet of water flowing per second from each square mile of area drained, assuming the runoff is distributed uniformly in time and area.

Inches (INCHES) indicate the depth to which the drainage area would be covered if all of the runoff for a given time period were uniformly distributed on it.

10 PERCENT EXCEEDS.--The discharge that has been exceeded 10 percent of the time for the designated period.

50 PERCENT EXCEEDS.--The discharge that has been exceeded 50 percent of the time for the designated period.

90 PERCENT EXCEEDS.--The discharge that has been exceeded 90 percent of the time for the designated period.

Identifying Estimated Daily Discharge

Estimated daily-discharge values published in daily-value tables are identified by flagging individual daily values with the letter symbol "e" and a footnote.

Data for partial-record discharge sites are presented in two tables. 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 generally are made in times of drought or flood to give better areal coverage to those events. Tables for partial-record sites include data for water years 1998 to current.

Annual maximum peak discharges and their associated stage values for the period of record is provided in this report under "Peak flow", which is available on the "Surface-Water Data" page and the "Available Data" page of each gaging station. Also, secondary instantaneous peak discharges and their associated stage values above a selected base discharge are available if the flow above the gage is not appreciably regulated. The base discharge generally is selected such that, on average, three secondary  peak-flow discharges, including the annual maximum peak discharge, will exceed the base discharge each water year. Also, if the maximum peak stage occurred at a different time than the annual maximum peak discharge, it also is listed. The specific criteria for (1) deciding which gaging stations should have secondary peaks, (2) selecting the base discharge, and (3) selecting secondary peaks greater than the base discharge are given in Novak (1985).

The peak-flow data are presented in text files. Columns of data in each file include: water year, date and time of peak, peak discharge, peak-flow qualification codes, gage height associated with the peak flow, gage-height qualification codes, year peak flow was last exceeded ("highest since"), maximum gage height (if different from associated gage height of annual peak flow), date and time of maximum gage height, gage-height qualification codes associated with the maximum gage height, and number of secondary peaks.  More information about qualification codes are provided as a link ("Peak-Flow Codes") on the "Peak-Flow" station-list page of the report.

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 FOR CURRENT YEAR." "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.

Values of daily mean discharges in this report typically 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 sites.

Discharge at many stations may not reflect natural runoff because of the effects of diversion, consumption, regulation by storage, increase or decrease in evaporation because of 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.

Records of water use from Lake Michigan are collected by the Illinois Department of Natural Resources, Office of Water Resources. These records may be obtained from the Illinois Department of Natural Resources, Office of Water Resources, 100 West Randolph St., Suite 5-500A, James R. Thompson Center, Chicago, IL 60601 (telephone number 312-793-3123).

Records of discharge, not published by the USGS, are collected at various sites in Illinois by the U.S. Army Corps of Engineers. The National Water Information System (NWIS) of the USGS contains an index of these sites as well as an index of records of discharge collected by other agencies but not published by the USGS.

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 Water Science Center (Science Center) whose address is given on the section titled "Access to USGS Water Data"  Information on the availability of the unpublished information also may be obtained from the Science Center.

Records of surface-water quality typically include physical, chemical, biological, microbiological, and sediment data. These records ordinarily are obtained at or near surface-water gaging stations because interpretation of records of surface-water quality often require corresponding discharge data. Records of surface-water quality in this report may involve a variety of types of data and measurement frequencies.  Annual data tables of surface-water quality are given in this report for water years 1998 to current.  In addition, daily values of sediment data (period of record) are provided in text files for continuous-record sediment sites.

Water-quality data for surface-water sites are grouped into one of three classifications. A continuous-record station is a site where data are collected on a regularly scheduled basis. Sample frequencies 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 usually is less than quarterly. A miscellaneous sampling site is a location other than a continuous- or partial-record station, where less frequent samples are collected to allow for better areal or temporal coverage to define water-quality conditions in a river basin. 

A careful distinction needs to be made between continuous records as used in this report and continuous recordings that refer to a continuous graph or a series of discrete values recorded at short intervals.  Some records of water quality, such as temperature and specific conductance, may be obtained through continuous recordings; however, because of costs, most data are obtained only monthly or less frequently.  Locations of stations for which 2004 records on surface-water quality appear in this report are shown in figure 5.  

To ensure that water-quality data represent the ambient quality of the water, certain measurements, such as water temperature, specific conductance, pH, and dissolved oxygen, must be made on-site when the samples are taken. To assure sample integrity, 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 given in Techniques of Water-Resources Investigations (TWRI), book 1, chap. D2; book 3, chap. C2; book 5, chap. A1,A3, and A4; book 9, chap. A1-A9; Shelton (1994); Horowitz and others (1994); and other USGS publications. Most of the methods used for collecting and analyzing water samples are described in the TWRIs, which may be accessed from http://water.usgs.gov/pubs/twri/. 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 Water Science Center. For quality-control procedures, refer to the section "Quality Control of Water-Quality Samples". 

One stream sample can define adequately the water quality at a given time if the mixture of constituents throughout the stream cross section is homogeneous. However, concentrations of constituents at different locations in the cross section may vary widely as a result of turbulence, different rates of water discharge, source of constituents and mixing of the stream. Most streams must be sampled in various vertical sections to obtain a representative sample. Whether samples are obtained from the centroid of flow or from various verticals depends on flow conditions and other factors that must be evaluated by the collector.

Water-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 USGS National Water-Quality Laboratory in Denver, Colorado and the USGS Organic Geochemistry Research Laboratory in Lawrence, Kansas.

For stations equipped with digital water-quality monitors, the records consist of daily maximum, minimum, and mean values for each constituent measured and generally are based on hourly values.

Water temperatures are measured at most 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 effluent 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 Water Science Center.

At continuous-record sediment stations, suspended -sediment concentrations are determined from samples collected with depth-integrating, isokinetic samplers at single-vertical locations, and/or with automatic water samplers collecting samples from a fixed point. Periodic cross sections are obtained at various verticals with depth-integrating, isokinetic samplers to compare to and adjust the single-vertical samples and/or the fixed point samples to compute the mean concentration of the cross section. Annual data tables show daily water discharge, suspended-sediment concentration and suspended-sediment discharge. Daily values of sediment concentration and sediment discharge also are provided in text files for the period of record.

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 loads 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 which can be accessed at http://water.usgs.gov/pubs/twri/. These methods are consistent with ASTM standards and generally follow IOS standards. For quality-control procedures, refer to the section "Quality Control of Water-Quality Samples". Locations of sediment stations for which water year 2004 data appear in this report are shown in figure 6.

At partial-record stations, suspended-sediment samples were collected periodically 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 predicting 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 for the National Water-Quality Assessment Program study units in the USGS Illinois Water Science Center are analyzed at the USGS sediment laboratory in Rolla, Missouri.  Sediment samples for other projects are analyzed at the USGS sediment laboratory in Louisville, Kentucky. Methods used in analyzing sediment samples and in computing sediment records are given in TWRI book 5, chapter C1 (http://water.usgs.gov/pubs/twri/). Samples for indicator bacteria (microbiological) are analyzed locally. Water-chemistry samples are analyzed at the USGS National Water-Quality Laboratory in Denver, Colorado, the USGS Geochemistry Research Laboratory in Lawrence, Kansas, or the Illinois Environmental Protection Agency Laboratory in Champaign, Illinois. 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 found in data tables of this report identify the agency or office having the principal responsibility for collecting and (or) analyzing water samples.
 

The USGS National Water-Quality Laboratory collects quality-control data (refer to: "Quality Control of Water-Quality Samples") on a continual basis to evaluate selected analytical methods to determine long-term method detection levels (LT-MDL's) and laboratory reporting levels (LRL's). These values are re-evaluated each year based on the most recent quality-control data and, consequently, may change from year to year.

Analytes detected at concentrations between the LT-MDL and LRL and that pass identification criteria are estimated. Estimated concentrations will be noted with a remark code of "E" (refer to "Remarks Codes" section). These data should be used with the understanding that their uncertainty is greater than that of data reported without the "E" remark code.

Dissolved trace-element concentrations traditionally have been reported at the microgram per liter (ug/L) level.  Recent evidence, mostly from large rivers, indicates that actual dissolved-phase concentrations for a number of trace elements are within the range of 10's to 100's of nanograms per liter (ng/L).  Data greater than the micrograms per liter (ug/L) level should be viewed with caution.  Such data actually may represent elevated environmental concentrations from natural or human causes; however, these data could reflect contamination introduced during sampling, processing, or analysis.  To confidently produce dissolved trace-element data with insignificant contamination, the U.S. Geological Survey began using new trace-element protocols at some stations in water year 1994 (Horowitz and others, 1994).

For continuous-record stations, information pertinent to the history of station operation is provided in station descriptions preceding the annual data tables. These station descriptions give details regarding location, drainage area, period of record, type of data available, instrumentation, general remarks, cooperation, and extremes for parameters currently measured daily. Station descriptions also are given separate from the annual data tables.

The following surface-water quality information, as appropriate, is provided in station descriptions:

LOCATION.--See Station Description under "Discharge and Stage;" same comments apply.

DRAINAGE AREA.--See Station Description under "Discharge and Stage;" same comments apply.

PERIOD OF RECORD.--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. Starting in water year 2000, the period of record contains a list of the various data types published with their corresponding period(s) of record.

INSTRUMENTATION.--Information on instrumentation is given only if a water-quality monitor, temperature recorder, automatic water sampler, or other sampling device in operation at a station.

REMARKS.--Remarks provide long-term information pertinent to the collection, analysis, or computation of the records.

REMARKS FOR CURRENT YEAR.--Established in water year 2000 and contains information pertinent to the collection, analysis, or computation of the current year's records.

COOPERATION.--Records provided by a cooperating organization or obtained for the USGS by a cooperating organization are identified here.

EXTREMES FOR PERIOD OF RECORD or CURRENT YEAR.--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 water-quality records are discovered after publication, appropriate updates are made in the U.S. Geological Survey's distributed data system, NWIS, and, subsequently, to its Web-based national data system, NWISWeb (http://water.usgs.gov/nwis/nwis). Users are encouraged to obtain all required data from NWIS or NWISWeb to ensure the most recent updates. Updates to NWISWeb currently are made on an annual basis.

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".  Each site is published with its own station number and name in the regular downstream-order sequence. Station descriptions are not provided with the data tables, instead, they are shown in the section titled "Station Descriptions."

Annual data tables of ground-water levels for water years 1998 to current are published in this report.  In addition, the period of record for ground-water level sites with greater than 5 years of record are published in text files.  Furthermore, the period of record for sites established after 1998 are also included in text files regardless of the number of data years available.  Locations of wells for which water-level data for water year 2004 given in this report are shown in figure 7.

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 consistently are accurate and reliable.

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 station description. The height of the measuring point (MP) above or below land-surface datum is given in each station description. The method and frequency of measurement also 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 500-1,000 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 better. Accordingly, most measurements are reported to a hundredth of a foot, but some are given to a tenth (or larger) of a foot.

Annual data tables of ground-water levels are presented by station number or county. The primary identification number for a given well is the 15-digit station number consisting of latitude, longitude, and sequence number as described in the "Latitude-Longitude System" earlier in this report.  The secondary identification number is the local well number, an alphanumeric identifier, derived from the township-range location of the well.

Annual data tables consist of two parts: the station description and the data table of water levels observed during the water year. The station description of a well precedes the data table. Station descriptions also are provided separate from the data tables.

Water levels are reported in feet below land-surface datum. For sites without recording equipment, all taped measurements of water level are listed. For wells equipped with recorders, abbreviated tables are published through water year 2002; (generally, only water-level lows are listed for every fifth day and at the end of the month (eom)); beginning in water year 2003, daily water-level lows are listed, as are end of the month statistics.  Prior to water year 2003, the highest and lowest water levels of the water year and their dates of occurrence are shown below the abbreviated tables. Beginning in water year 2003, the highest and lowest water levels of the water year and their dates of occurrence are shown below the tables of sites with hand-taped measurements and above the tables of sites with recording equipment.  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.

The following ground-water-level information, as appropriate, is provided in station descriptions:

LOCATION.--Reports the latitude and longitude (generally in degrees, minutes, and seconds) with the associated national horizontal datum in parentheses; a landline location designation; the hydrologic-unit number; a local well name, if applicable; the distance and direction from a geographic point of reference; and the owner's name.

AQUIFER.--Designates the name and geologic age of the aquifer(s) open to the well.

WELL CHARACTERISTICS.--Describes the observation 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.--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.--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 feet above land-surface datum). The elevation of the land-surface datum is described in feet above (or below) associated national vertical datum; it is reported with a precision depending on the method of determination.

REMARKS.--Describes long-term factors that may affect water levels in a well or measurements of water levels. It should identify wells that also are water-quality monitoring wells.

PERIOD OF RECORD.--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. Starting in water year 2000, the period of record contains a list of the various data types published with their corresponding period(s) of record.

EXTREMES FOR PERIOD OF RECORD or CURRENT YEAR.--Contains the highest and lowest water levels of the period of published record or current year, with respect to land-surface datum, and the dates of their occurrence.

Records of ground-water quality in this report differ from other types of records in that, for most sampling sites, records consist of generally one set of measurements for each water year. The quality of ground water ordinarily changes slowly in relation to surface water; therefore, samples for water-quality analyses are taken from wells on an infrequent (annually or longer) basis. In special cases, where ground-water quality may change more rapidly, or if a particular problem is of concern (such as monitoring for trends in nitrate concentration), more frequent measurements are made as needed to quantify these changes.

Annual data tables of ground-water quality for water years 1998 to current are published in this report.  Locations of wells for which water-quality data for water year 2004 are given in this report are shown in figure 8.

Most methods for collecting and analyzing water samples are described in the TWRIs referred to in the "On-site Measurements and Sample Collection", and the "Laboratory Analyses" sections in this report.  In addition, TWRI Book 1, Chapter D2, describes guidelines for the collection and field analysis of ground-water samples for selected unstable constituents.  Procedures for on-site measurements and for collecting, treating, and shipping samples are given in TWRIs Book 1, Chapter D2; Book 5, Chapters A1, A3 and A4; and Book 9, Chapters A1 through A6.  The TWRI publications may be accessed from http://water.usgs.gov/pubs/twri/.  The values in this report represent water-quality conditions at the time of sampling, as much as possible, and that are consistent with available sampling techniques and methods of analysis.  These methods are consistent with ASTM standards and generally follow ISO standards.  Trained personnel collected all samples.  The wells sampled were pumped long enough to ensure 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, composing the casings.

The records of ground-water quality are presented by station number or by county. The station number for wells sampled is the 15-digit station number derived from the latitude-longitude locations as described in the "Latitude-Longitude System" presented earlier in this report.

The station description of a well precedes the data table. Each heading in the station description provides the same information as presented in the "Ground-Water-Levels" section. Station descriptions also are provided separate from data tables in the section titled "Station Descriptions."  The "Remark Codes" listed near the end of this introductory text also may be applicable to ground-water-quality records.

Meteorological records in this report consist of precipitation, air temperature, dew point, solar radiation, and wind speed. Non-precipitation records (air temperature, dew point, solar radiation, and wind speed) are discussed in the "Other Meteorological Records" section. Locations of meteorological sites for which data for water year 2004 are given in this report are shown in figure 9.

Precipitation

Annual daily-value tables of 24-hour precipitation totals in and near DuPage County for water years 1986 to current are published in this report where approved and available in the USGS Illinois Water Science Center's computer system. Starting in water year 2000, other selected precipitation sites also are published. Daily values are given in text files.  Depending on the type of instrumentation used to collect precipitation data, gage values of instantaneous (0.01-inch increments) or 5-minute totals of 0.01-inch increments are provided in text files where available.  

Collection and Computation

Rainfall values are obtained by using tipping-bucket rain gages with electronic data loggers. Collection, computation, and publication of precipitation data do not necessarily conform to standards used by the National Weather Service. Precipitation data could be subject to errors for days with freezing and/or thawing of precipitation.

For DuPage County rain-gage sites, 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 from field observations and by comparing recorded data with snowfall, snow depth, and temperature and precipitation data from the National Oceanic and Atmospheric Administration precipitation gages. For the period of record between water years 1997 and 2000, 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). For water year 2001 to present, daily values for periods of missing record were estimated from the closest rain gage.

Presentation of Annual Data Tables

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 gaging station is not available, the precipitation record is published with its own name and 15-digit latitude-longitude identification number.

Information pertinent to the history of a precipitation station is provided in station descriptions that precede the data table. These station descriptions give details regarding location, period of record, record accuracy, general remarks, and maximum daily values. Station descriptions also are provided separate from data tables.

The following precipitation-station information, as appropriate, is provided in station descriptions: 

LOCATION.--Reports the latitude and longitude (generally in degrees, minutes, and seconds) with the associated National horizontal datum in parentheses. Information on locations is obtained from the most accurate maps available. The latitude and longitude of a station is determined from maps and/or global-positioning systems.  A consideration used in the determination of site locations was the proximity of existing structures and minimal obstructions.

PERIOD OF RECORD.--See Station Description section under " Discharge and Stage;" same comments apply.

GAGE.--Description of the gage equipment in current use.

REMARKS.--Provides long-term information pertinent to the collection, analysis, or computation of meteorological records.

EXTREMES FOR PERIOD OF RECORD.--Maximum daily precipitation value for the period of record.

EXTREMES FOR CURRENT YEAR.--Maximum daily precipitation value for the current year.

Other meteorological records in this report consist of air temperature, dew point, solar radiation, and wind-speed measurements for one station at the Argonne National Laboratory (ANL). The measurements were made by ANL staff. 

Collection and Computation

In more recent years, the ANL used standard automated meteorological instruments. Older data were recovered from paper records and digitized. All data underwent a checking procedure at ANL and then were posted on the Web (http://www.atmos.anl.gov/ANLMET/). The USGS obtained these data, performed additional checks, filled in missing periods using data from adjacent National Weather Service and Illinois Climate Network stations, and by comparison with these data from adjacent stations, adjusted historical data to make it statistically homogeneous with current measurements. 

Presentation of Annual Data Tables

Data are presented as series of hourly values in text files. Associated with each value is a three-digit, data-remarks code that explains the origin and transformations applied to the value. An explanation of the remark codes are provided as a link in the "Meteorological Data" section of this report.

Since water year 1989, stream biology and physical habitat data have been obtained by the National Water-Quality Assessment (NAWQA) Program study units in the USGS Illinois Water Science Center. The NAWQA Program collects in-stream algae, benthic macroinvertebrate, fish and habitat data. Annual data tables of algae for water years 1989 to current are published in this report.  Annual-data tables and text files of benthic macroinvertebrate, fish, and habitat data for water years 1996 to current are also published in this report. Locations of biological samples given in this report are shown in figure 10.

Biological community samples and habitat data were collected since 1996 according to protocols established by the NAWQA Program (Cuffney and others, 1993; Meador and others, 1993; Porter and others, 1993; Fitzpatrick and others, 1998, Moulton and others, 2002). Macroinvertebrate and algae sampling often include both quantitative and qualitative samples. All data were collected within defined reaches, or lengths of stream, which are at or near surface-water quality stations. When multiple samples were collected, they were collected from separate reaches near a station. Letter designations were given to these reaches with "A" being the most upstream reach.

The method of sample processing depended on the type of organism sampled. Fish primarily were identified in the field with selected specimens collected for identification, verification, or vouchering at the Illinois Natural History Survey, Champaign. Macroinvertebrate and algal samples were sent to the USGS National Water-Quality Laboratory for processing, enumeration and taxonomic identification. Habitat characteristics were measured both in the field and calculated in the office.

Annual tables of algae include periphyton, pigment (chlorophyll), and biomass.  Annual-data tables for biological community data (benthic macroinvertebrate and fish) are arranged by taxonomic group. Habitat data are collected hierarchically (in streams reaches and along cross sections within a reach), and are presented in multiple files. Information pertinent to the history of a biological station is provided in the station description that precedes the annual-data table. These station descriptions give details regarding location, period of record, drainage area and general remarks. Station descriptions also are provided separate from data tables.