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National Water Information System: Web Interface

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ICE EFFECTS ON STREAMFLOW

The formation of ice on rivers can cause discharge values to appear unusually high. Display of these erroneous discharge data may result in improper assessment of flow conditions and misuse of the data. For this reason, display of discharge values for streams significantly affected by ice may be disabled from view. Display of discharge data will resume when ice conditions are no longer present. Discharge values for streams minimally affected by ice will continue to be displayed. Flows for streams with these conditions appear to increase during the night and decrease to near-base-line conditions around midday. To estimate the correct discharge for these streams, use the flow rate that corresponds to the bottom of the discharge curve, rather than the peak that corresponds to the top of the curve. Note that this method of estimation is possible only when no surface runoff is occurring. Daily mean discharges for periods of both significant and minimal ice-effect will be estimated and published in the Annual Water-Data Report for the water year in which they occurred.

Current and Historical Instantaneous Data Availability

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.