Stream gages and hydrographs

In my ramblings about weather stations, I mentioned that stream gages are another type of automated data recorder that provide real-time data to the public. Stream gages record the height of the flowing water above a reference datum (such as a particular elevation above sea level), termed the stream’s stage. Stage measurements have a units of length, such as 7.31 feet. Stream stage is not a particularly useful measurement on its own; however, stream discharge is extremely useful information. Discharge is the volume of water that passes through a given area of stream in a measured period of time, reported in units of volume per unit time, such as cubic feet per second. Workers periodically measure the dimensions and discharge of the stream at each gage in order to make a stage-discharge relation curve. This stage-discharge relation allows estimation of stream discharge from stream stage measurements made at the gage. Stream gages measure stage, which is then converted to discharge using the stage-discharge relation curve. Stage-discharge relation curves are often adjusted several times during a year. Gages record stage instead of directly measuring discharge because measuring discharge directly is much more difficult.

A stage-discharge relation curve for the stream gage on the Salt River at Priest Drive in Tempe, Arizona

The US Geological Survey makes available real-time stream gage data on their website. Data is available in both tabular and graphical formats. An interactive map helps locate appropriate gages in the stream of interest. While most people probably find this data unimportant and boring, investigators glean important information from steam gage data. Stream discharge data is crucial for:

– warning of flash floods;
– calculating flood crests;
– understanding river flow and sediment transport;
– delineating flood plains and construction of flood hazard maps;
– design, management, and operation of dams;
– bridge design;
– adjudication and allotment of water for water users,
– shipping and other commerce that rely on waterways;
– and other studies relating to stream and river ecosystems.

Stream discharge information is also useful to many in the general public, who use waterways for boating or fishing, or who cross streams for land access.

The Salt River gage at Priest Drive. Image from the Maricopa County Flood Control District.

A hydrograph is a graphical representation of a water property with respect to time. I downloaded discharge data from the Salt River gage at Priest Drive in Tempe, Arizona, and constructed a hydrograph for July 20 and 21, at the time the Tempe Town Lake dam ruptured. On the vertical axis I plotted river discharge as reported by the gage, and time on the horizontal axis.

The hydrograph shows the baseline discharge prior to the dam rupture of about 20 cubic feet per second (cfs). The peak flood discharge recorded at the gage is about 25,900 cfs. The dam broke approximately 9:55PM, the peak discharge occurred about 10:05PM and elevated discharge lasted until about 2PM on July 21st , when the river flow returned to approximately the baseline value. After 2pm on July 21, discharge rose approximately 200cfs, possibly due to increased discharge from a wastewater treatment plant, or because of a precipitation event. Town Lake drained entirely in about 16 hours.

Unfortunately, many stream gauges are no longer maintained dues to lack of funding. The resulting loss of data undermines future studies of stream dynamics and analysis of potential flood hazards.

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One Response to Stream gages and hydrographs

  1. Dave says:

    By the sounds of it – you have supported gages.

    Me too. On the Colorado River.


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