There are Many Ways to Measure Stream Flow

There are Many Ways to Measure Stream Flow

Have you got an orange? The classic method is to throw an orange into the stream and time how long it takes to float from point A to point B. You then measure the average width and average depth of the stream, do some math and voila! you have a flow measurement. Obviously, this isn’t the most accurate method. For one thing, the flow at the surface isn’t the same as the flow along the bottom or in the middle or along the banks of the stream.

If you want to refine your measurement you can use a propeller. The speed of the propeller is proportional to the flow. This method is very accurate when the propeller is installed in a pipe. Used in a stream, you take readings below the surface at multiple locations across the stream. You write down all your readings, do some math and you get a more accurate flow measurement than by using an orange. But propellers can be cranky. The bearings get sticky, the blades get dinged. Or maybe you don’t hold it exactly perpendicular to flow at each point of measurement. And each reading must be recorded in a notebook which creates the possibilities of transcription errors. And then there’s the math. Lots of room for error there!

The next level of accuracy is an acoustic doppler current profiler (ADCP). This is a device that sends out an ultrasonic audio signal into the water and listens for the echo. The echo will be slightly lower or higher in pitch depending on the flow of the water. Imagine the example of a car approaching you with its horn blaring. As it gets closer the horn seems to increase in pitch. As it passes you and then drives off the horn seems to lower in pitch. The ADCP does this on a very small scale.

A sophisticated ADCP also tells you where to place the probe, records the results, then does the math for you! Brilliant. But as you might imagine just by the name, these things are pricey. Fortunately for BWS, stream flow is an important parameter to the BC Conservation Foundation (BCCF). The Tsolum River Restoration Society made a successful grant application to the Pacific Salmon Foundation to purchase a FlowTracker, a commercial ADCP. The intent was to have it available to other stream keeper groups. BWS has taken full advantage of this arrangement. Recently, two BWS members joined Thea Rodgers from BCCF and two Ministry of Environment hydrologists at Cook Creek and Wilfred Creek for some training.

The FlowTracker is a super sensitive instrument, but accurate measurements still involve a lot of human input. You must still choose a good site (with minimal turbulence, no backwash, no eddies, as few big rocks as possible). You must learn to read the currents. You hear it often when hanging out with hydrologists: flow measurement is as much art as science. Spending the day with experts really emphasized the limits of fancy technology and the importance of training and experience. The trick is to eliminate as many external variables as possible. We want consistent readings from one visit to the next, from one creek to another. We want to be able to compare apples to apples and oranges to oranges.