- 1). Set up two transects of the stream or river. Stretch a length of string from one bank to the other, tying it to branches or trees or securing it with rocks. Place the second transect downstream, at least 20 feet from the first one. These are the start and finish lines for your velocity measurement.
- 2). Measure the distance from one transect to the other and record the result.
- 3). Toss an orange into the stream, a few feet upstream of the first transect. An orange is well suited to this type of measurement because it is heavy enough to float mostly under the surface of the water, so it is subject to the full force of the surface current. If you lose it, it will also biodegrade without harming the stream.
- 4). Start the stopwatch just as the orange passes under the first string. Stop it when the orange passes under the second string.
- 5). Catch the orange with a net and repeat Steps 3 and 4 several times. The more trials you run, the more accurate your result. Try to get the orange to land the same distance from the bank every time, because surface velocity can vary significantly from the edge of the bank to the center of the stream. It's not a bad idea to measure velocity at different places in the stream, but don't get your times mixed up. If the orange gets held up by rocks or debris in the stream, discard that trial and try again.
- 6). Add up all the recorded times and divide by the total number of trials. This gives the average time it takes for the orange to go from one transect to the other. If one of your times is radically different from the others, make sure that the orange wasn't obstructed and that it was the same distance from the bank as in the other trials.
- 7). Divide the distance between transects, as measured in Step 2, by the average time it took the orange to go from one to the other. The result is your velocity, in whatever units you used to measure. If you measured in feet, the result is in feet per second. If you measured in meters, it is in meters per second.
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