Stream flow measurement without a flow meter

If you do not have access to a current meter but still wish to obtain a rough estimate of stream flow, you can do so by floating a buoyant object down a segment of stream. Oranges or partially filled plastic water bottles work well for this. However, because the object will float the surface of the water, we will need to make an adjustment to our measurement in order to estimate the stream's average velocity.

To take the measurement:

1. Measure off a straight segment of stream at least 50 feet long.
2. Estimate the cross-sectional area of the stream:

• Measure the width of the stream using a measuring tape.
• Measure the depth of the stream at 1-foot or 2-foot intervals using a yardstick.
• Calculate the average depth from these measurements:

Total width (ft) x Average depth (ft) = cross-sectional area (ft 2)

Using a stopwatch, measure the time it takes for the float to cover the 50-foot section of stream. Repeat the measurement several times (typically three times) and calculate the average time.

1. Calculate the stream velocity:

Velocity = Distance traveled / time to travel distance

For example, if it took 45 seconds for the float to travel 50 feet, then the stream velocity is 1.1 ft/seconds:

50 feet/45 sec = 1.1 ft/sec

Next, we must correct for the fact that we took our measurement at the stream surface, instead of in the middle of the water column, by multiplying the surface velocity by 0.85:

ft/sec x 0.85 = 0.935 ft/sec

Finally, we can calculate the stream discharge in cubic feet per second (cfs) by multiplying the stream velocity (ft/sec) by the cross-sectional area (ft 2) of the stream:

0.935 ft/sec * 14.4 ft 2 = 13.46 cfs