Editing Stream gauge (section)

==Measurement equipment==
{{See also|Streamflow#Measurement}}
[[Image:Snoqualmie Gaging Station 04135.JPG|thumb|upright|Stream Gaging Station, [[Carnation, Washington]]]]
Automated direct measurement of stream discharge is difficult at present. Mathematically, measuring stream discharge is estimating the [[volumetric flow rate]], which is in general a [[flux integral]] and thus requires many cross-sectional velocity measurements. In place of the direct measurement of stream discharge, one or more surrogate measurements can be used as [[proxy variable]]s to produce discharge values. In the majority of cases, a stage (the elevation of the water surface) measurement is used as the surrogate. Low gradient (or shallow-sloped) streams are highly influenced by variable downstream channel conditions. For these streams, a second stream gauge would be installed, and the slope of the water surface would be calculated between the gauges. This value would be used along with the stage measurement to more accurately determine the streamflow discharge. Improvements in the accuracy of velocity sensors have also allowed the use of water velocity as a reliable surrogate for streamflow discharge at sites with a stable cross-sectional area. These sensors are permanently mounted in the stream and measure velocity at a particular location in the stream.

In those instances where only a stage measurement is used as the surrogate, a '''[[rating curve]]''' must be constructed. A rating curve is the functional relation between stage and discharge. It is determined by making repeated discrete measurements of streamflow discharge using a [[Velocimetry|velocimeter]] and some means to measure the channel geometry to determine the cross-sectional area of the channel.  The technicians and [[hydrologist]]s responsible for determining the rating curve visit the site routinely, with special trips to measure the hydrologic extremes (floods and droughts), and make a '''discharge measurement''' by following an explicit set of instructions or standard operating procedures (SOPs).

[[Image:HueyCreekAntarctica2001.jpg|thumb|December 12, 2001 photo of the [[USGS]] streamflow-gaging station at [[Huey Creek]], [[McMurdo Dry Valleys]], [[Antarctica]].]]
 	
Once the rating curve is established, it can be used in conjunction with stage measurements to determine the volumetric streamflow discharge.  This record then serves as an assessment of the volume of water that passes by the stream gauge and is useful for many tasks associated with hydrology.

In those instances where a velocity measurement is additionally used as a surrogate, an '''index velocity determination''' is conducted. This analysis uses a velocity sensor, often either magnetic or acoustic, to measure the velocity of the flow at a particular location in the stream cross section. Once again, discrete measurements of streamflow discharge are made by the technician or hydrologist at a variety of stages.  For each discrete determination of streamflow discharge, the mean velocity of the cross section is determined by dividing streamflow discharge by the cross-sectional area. A rating curve, similar to that used for stage-discharge determinations, is constructed using the mean velocity and the index velocity from the permanently mounted meter. An additional rating curve is constructed that relates stage of the stream to cross-sectional area. Using these two ratings, the automatically collected stage produces an estimate of the cross-sectional area, and the automatically collected index velocity produces an estimate of the mean velocity of the [[Cross section (geometry)|cross section]].  The streamflow discharge is computed as the product of the estimate of the cross section area and the estimate of the mean velocity of the streamflow.

[[Image:B62doddieburn.jpg|thumb|Stream gauge B62, a [[Weir#Compound weir|combination weir]] at Doddieburn, on the [[Mzingwane River]], [[Zimbabwe]]]]

A variety of hydraulic structures / primary device are used to improve the reliability of using water level as a surrogate for flow (improving the accuracy of the rating table), including:
* [[Weir]]s
**[[Weir#V-notch weir|V-notch]],
** [[Weir#Broad-crested weir|broad-crested]],
** [[Weir#Sharp crested weir (fayoum weir)|sharp-crested]] and
** [[Weir#Compound weir|combination]] weirs
* [[Flume]]s
**[[Parshall flume]]

Other equipment commonly used at permanent stream gauge include:
* Cableways - for suspending a hydrographer and current meter over a river to make high flow measurement
* Stilling well - to provide a calm water level that can be measured by a sensor

{{anchor|Water level}}Water level gauges:
* [[Staff (head) gauge]]s - for a visual indication of water depth
* Water pressure measuring device (Bubbler) - to measure water level via pressure (typically done directly in-stream without a stilling well)
* Stage [[rotary encoder|encoder]] - a [[potentiometer]] with a wheel and pulley system connected to a float in a stilling well to provide an electronic reading of the water level
* [[Ultrasonic sensor|Simple ultrasonic devices]] - to measure water level in a stilling well or directly in a canal.
* Electromagnetic gauges

[[Discharge (hydrology)|Discharge]] measurements of a stream or canal without an established stream gauge can be made using a [[current meter]] or [[Acoustic Doppler current profiler]]. One informal methods that is not acceptable for any official or scientific purpose, but can be useful is the '''float method''', in which a floating object such as a piece of wood or orange peel is observed floating down the stream.<ref>{{cite web|url=http://www.fs.fed.us/rm/pubs_rm/rm_gtr245.pdf|title=Stream channel reference sites: an illustrated guide to field technique. Gen. Tech. Rep. RM-24|publisher=U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station|author1=Harrelson, Cheryl C |author2=Rawlins, C. L. |author3=Potyondy, John P. |year=1994|access-date=2011-01-13}}</ref>