USGS Water Data for Illinois
The available parameters from the Current Conditions and Historical Observations pages on the National Water Information System Web Interface have two categories of instantaneous data availability. For some parameters, historical data are available as far back as October 1, 1950, whereas for others only the most recent 120 days of provisional data are available. The period of availability depends on the intended use of the data and can vary from site to site and/or between parameters at a site as described below.
Available Historical Instantaneous Data:
Parameters with historical instantaneous data are measured by the USGS Water Science Center (WSC) with the intent to publish and are subject to a thorough review and approval process. Typically, these data have corresponding historical daily values available for the period of record. These instantaneous data are limited to data collected since October 1, 1950.
Data are presented as approved or PROVISIONAL as appropriate, however provisional data will be provided for only the most recent three years. In cases where data are not approved within a three year period, interested users should contact the local WSC for further information.
Available Operational Instantaneous Data:
These parameters typically are collected by the USGS for the purposes of supporting short-term operational needs of the data network (for example precipitation at some locations) or parameters measured to facilitate the computation of another parameter of interest (for example, stage is used in the computations of streamflow). In either case, these parameters are not necessarily corrected for errors or omissions either because the short-term need for the operational data has passed or because other methods were used to estimate the primary parameter of interest. Operational instantaneous data typically are not intended for publication and therefore are limited to the most recent 120 days.
Please note that the above designations are not uniformly applied from site to site for a given parameter. For example, whereas stage data at some sites are fully quality assured, published, and are historically available, at other sites they are not quality assured and will only be available for 120 days. The same site-to-site variations will exist for precipitation and other parameters depending on the intended uses of the data.
Significant gaps in the availability of historical instantaneous data may exist due to instrument problems, environmental conditions or other factors that affect the ability to collect data. For some parameters, especially streamflow and others that are routinely estimated as necessary, a complete record may be available as Daily Data.