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About IDA

Since 1889 the United States Geological Survey has collected continuous discharge and other time-series data on the nations rivers and streams, with intervals commonly ranging from 5-60 minutes. Historically, these instantaneous data have been processed into various daily values, such as the daily maximum, minimum and/or mean. This was done primarily to provide concise values for publication in paper reports (prior to the advent of the computer age) or storage without significant memory usage (early in the computer age). In more recent years, and particularly since the USGS began making real-time instantaneous data available on NWISWeb in 1994, more attention has been given to historical instantaneous data and USGS offices have received increasing requests for these data. There have been several challenges associated with meeting those requests, among them are that:

1. Most historical instantaneous data were never stored on a computer or were removed after the computation of the daily values in order to save space. In most cases this data still exists as original field records, but it is a significant effort to process back into electronic records.
2. Instantaneous data have not historically received the same level of quality assurance as the official daily values. For example, periods of ice effect commonly affect the base calculations of discharge at a stage-discharge station. These periods are typically cleaned up by estimating the daily values directly, but the corresponding instantaneous data is left unchanged and thus remains erroneous.
3. There may be several versions of instantaneous data available for the same period due to the presence of backup records or data from the same recorder transmitted via multiple means (such as by satellite and by telephone modem). These different data sets may vary in quality and it is often not clear which was used to calculate the published daily values for any given period.

In developing a system to make historical instantaneous data available, the USGS has attempted to balance the fact that it is not possible to completely quality assure historical instantaneous data at most site due to the time and resources required, with the fact that it would not be appropriate to simply show all available data and confuse the user with multiple data sets of ambiguous data quality. Thus the archive as been built around a process that compares the available instantaneous data to the published daily values. For each set of instantaneous data that exists for a given day, a new daily value is calculated and compared to the published daily value. The instantaneous data set that computes a value that most closely matches the published value is added to the archive for that day, along with an accuracy code explaining how closely it matched. Currently the archive is limited to comparing discharge data only against the published daily mean values. The possible classifications are as follows:

0 - A daily mean discharge calculated from the instantaneous data on this day is 0.01 cubic feet per second or less and the published daily mean is zero.
1 - A daily mean discharge calculated from the instantaneous data on this day matches the published daily mean within 1 percent.
2 - A daily mean discharge calculated from the instantaneous data on this day matches the published daily mean from greater than 1 to 5 percent.
3 - A daily mean discharge calculated from the instantaneous values on this day matches the published daily mean from greater than 5 to 10 percent.

If the daily mean discharge calculated from the instantaneous values on a given day is greater than 10 percent different from the published daily mean, those instantaneous values are excluded from the archive. In addition, instantaneous data that corresponds to a daily mean value that was estimated are automatically excluded from the archive regardless of any comparison.

It is important to note that the values available in the archive have thus not been individually reviewed and approved. They have been automatically compared against the published daily mean value and found to have no gross errors when used to compute a daily mean. Individual instantaneous values may still have significant error. For example a data spike might cause a given value to be off by several hundred percent, yet when combined with the other available values in the daily mean calculation the result might lead that spike to get added to the archive with an accuracy code of 2 or 3 because the impact on the daily mean is less than 5%. Users of the archive are thus strongly encouraged to review all data prior to use.

The USGS converted to electronic storage and computer processing of instantaneous time series data in the 1980's, however it was not until the late 1990's that computer storage was sufficient to hold long-term periods of instantaneous data. Recovery of historical data from backup tapes has been done in most locations, and thus stations available on this web site will typically have data starting in the late 1980s. Earlier data are archived on paper charts and must be processed manually. Populating this web site with instantaneous discharge data takes effort and resources that are being provided "as available." Because of this not all states and stations are available at this time; you should see the number of states and stations with data increase over time.