Determination of peak streamflows

Stream stage (or water level) and streamflow (or discharge) are measured at locations called streamflow gaging stations. The U.S. Geological Survey (USGS) often measures and records stage data continuously by electronic instruments to an accuracy of 0.01 foot.

Streamflow is more difficult to measure accurately and continuously than is stage. Streamflow for a gaging station is typically determined from an established stage-discharge relation, or rating curve. Individual discharge measurements are made by USGS personnel at a gaging station by using standard procedures (Rantz and others, 1982); ideally, these measurements are made when the stream stage is not changing. A series of these measurements made over a range of flow conditions defines the rating curve, which is used to convert continuous measurements of stage to a continuous record of discharge. Channel changes, resulting from scour, deposition, vegetation, or other processes, alter the stage-discharge relation, so that discharge measurements must be made routinely and continuously to ensure that the rating curve remains accurate.

Estimates of peak flows, which are outside the range of the established rating curve, may be made by an extrapolation of the rating curve to the peak stage. At some gaging stations, indirect methods of discharge determination based on high-water marks, channel properties, and hydraulic principles may be used to obtain an independent estimate of discharge. These indirect methods generally require accurate field surveys to determine high-water marks, channel properties, and channel shape. The information obtained in the field is then processed using computer programs to determine the discharge.

Information on this page is adapted from "Determination of peak flows" USGS Fact Sheet FS-036-98--April 1998. Effects of August 1995 and July 1997 Storms in the City of Charlotte and Mecklenburg County, North Carolina Web site. (July 8, 1998). [Online] Available at http://water.usgs.gov/pubs/FS/FS-036-98/text/determination.html [Accessed on May 22, 2001].

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