This Is How You Measure Stream Velocity Without Getting Your Feet Wet

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Stream gaging sure was a dapper occupation at the turn of the century! Here's the various techniques United States Geological Survey hydrologists used to measure current without even dipping a toe into the streams and rivers.

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Current flow measurement of the Arkansas River in 1890. Image credit: USGS

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These dapper lads are a pair of United States Geological Survey hydrologists measuring the current flow of the Arkansas River from a cable-suspended stream-gaging car in 1890. These hydrologists were hard at work measuring the velocity of water within the Arkansas River near Cañon City in Fremont County of Colorado.

Measuring Rum River water velocity from a suspended platform upstream of a millpond near Anoka, Minnesota. Image credit: USGS

Their photograph was included in the 1901 United States Geological Survey water supply paper 56, Methods of stream measurement, along with an entire collection of downright dashing scientists modelling field techniques. Other techniques described by author Frederick Haynes Newell include measuring from bridges, ferries, the shoreline, and even specialized dams called weirs.

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Stream measurement from a cable car above the Shenandoah River near Millville, West Virginia. Image credit: USGS

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Gaging station on Clear Creek in Wyoming. Image credit: USGS

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Stream gage measurement from a weir, a shallow dam above an open channel, in the Los Angeles River, California. Image credit: USGS

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Stream measurements from a suspension bridge over the Neshaminy Creek in Rush Valley, Pennsylvania. Image credit: USGS

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Using an Ellis meter from a bridge above the Mississippi River near Anoka, Minnesota. Image credit: USGS

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Measuring the Mississippi River from a boat and cable upstream of the mouth of the Crow Wing River in Minnesota. Image credit: USGS

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Measuring velocity from a ferryboat in the Tugaloo River near Fort Madison, South Carolina. Image credit: USGS

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Pulling a boat along a cable looks awfully celebratory on the Mississippi River in Saint Paul, Minnesota. Image credit: USGS

DISCUSSION

By
mrandreww

Oooh hey finally something I can contribute knowledge to.

Stream flow measurements (accurate ones, at least) require a couple of pieces of information: first, you need the shape of the channel that you're measuring, to provide you with the cross-sectional area (A), and then you need to measure velocity, somehow (V). There are a few different ways to achieve both, but the much more difficult measurement is velocity. Some of the older methods would use bubblers (if you know where the bubbles start, and you see where they show up on the surface, then you get a rudimentary velocity) or price meters, but this still has an issue of 'where the hell do I take the measurements?', because obviously velocity varies a lot across the channel. USGS came up with some standard spots to measure, and then you average them (probably a weighted average) to get your average channel velocity.

THEN, you apply the continuity equation, that stipulates that discharge is equal to area times velocity, or:

Q(m^3/s) = A (m^2) * V (m.s)

Modern methods of measuring flow are a bit more elaborate and elegant, but rely on the same basic principles. Probably the most accurate measurement method is using acoustic doppler current profilers (ADCP), which use acoustic pulses to measure both the channel geometry below the ADCP (there's a sharp echo at the channel bed, although sometimes the channel bed can be moving, which can get corrected by comparing what the ADCP measures as its movement from the bed and what is measured via GPS), and relies on the doppler shift caused by particles moving in the water, which happens to be at the same speed as the water.

One interesting aspect of using ADCPs is the big improvements in resolution over the past few years by using higher frequencies. Older models would provide a bin that was about 0.5 m ish deep, whereas newer ones can have bins about 1 cm deep.

Neat stuff. Feel free to ask questions!