Skip to main content

Data Processing

​Any initial computation of discharge will result in some form of "provisional data"; however, there are classes of "provisional" that represent differing qualities of values. Provisional data in any form are subject to change prior to publication. Because of the variety of factors that can introduce errors in provisional streamflow data, users are cautioned to consider carefully the nature of the information before using it for decisions that concern personal or public safety or the expenditure of money.

Preliminary Real Time Data

The information on near real time streamflows (same day) available at this web site are preliminary. The information has been calculated based on raw data with little or no data quality review. A computer regularly pings each of the stations to obtain information on stage and converts the information to discharge. This automated process does not allow confirmation of either the accuracy of the stage data or whether the stage-discharge relationship has been altered. Therefore, there may be large differences between the real time data provided at this and other web sites and true discharge. Major adjustments in the data may be made prior to publication of the record. If the data are used for any purpose, a disclaimer should be included that describes the provisional nature of the data and the potential for significant errors.

Other Preliminary Data

Data collected at each gaging stations are regularly reviewed by agency staff to verify the recorded stages and any changes in the stage-discharge relationship. Corrections or adjustments based on direct observations and discharge measurements are applied to compute the basic discharge record. Estimated values are developed for periods during which data are missing or determined to be not representative of true discharge. For example, if the recorded stage did not change during several days of cold weather and then dropped abruptly after warmer temperatures, staff would assume that ice had prevented movement of the float. The values for the period during which the float was frozen would be estimated based on known values at that station, runoff patterns at other nearby stations, and other available information.

If the initial evaluation of the data has been performed, these data will be representative, but not necessarily reflect final quantities. Users are cautioned that the review of the data to ensure accuracy may not have been completed. The data may be revised upon further analysis. A disclaimer indicating the provisional nature of the data should be included if the data are used for any purpose. 

Additional information about streamflow measurement protocols is available at the U.S. Geological Survey website. Questions about the Water Resources Department streamflow gaging program may be addressed to the Department´s hydrographics staff.

​After the conclusion of the water year (October 1 to September 30), the streamflow data, if preliminary, are again reviewed or, if discharge has not yet been determined, the data development and review process is initiated. During this process, any previous computations of streamflows are evaluated and adjustments are made as necessary to reflect changes in the stage-discharge curve that may have occurred during the year. The streamflow data from stations in the drainage basin are compared to those from the other stations in the same or nearby, to weather records to determine if the flow patterns are consistent. Estimates of streamflows are verified or developed where the data are missing or appear to be erroneous. All data are considered to be provisional and are subject to change until the review process has been completed.

When the review process has been completed, the streamflow data are considered final and are published. These published records may be cited in research and used for other purposes. However, on rare occasions, errors are discovered after publication. When errors are discovered, the data are corrected in the electronic files available on the web and revisions are published at the first opportunity.

The accuracy of streamflow records depends primarily on (1) the stability of the stage-discharge relation and the frequency of discharge measurements; (2) the accuracy of measurements of stage, measurement of discharge, and (3) the interpretation of the records.

​​Streamflow data for a gaging station are developed by regularly recording the height of the water surface (stage) at the station. Streamflow is calculated by converting the stage data to a discharge rating developed from measurements made at the site. 

A rating is a simple model that predicts how stream stage relates to discharge. It is developed by directly measuring flow at various stages. Flow measurements are performed by sampling velocity at certain depths and width intervals across a section of the stream. The flow rate in each interval is found by multiplying its width, depth, and velocity. The discharges for all the intervals are added together to get the total discharge of the stream.

Erosion and deposition in the stream channel near a gage can have significant and permanent effects on the cross-section of a stream. The cross section can also be affected temporarily by fallen trees, ice, and aquatic vegetation. Changes in the cross-section of a stream alter the relationship between stage and discharge. The development of accurate records requires that the stage-discharge curves be verified by periodically repeating the discharge measurements and defining the effects of changes on the stream channel.