If you don't have flow meter to measure the flow of your river or stream and rough estimate is adequate, you can measure flow by using a simple float.
Determine the Flow of Your Stream with Simple Float
If a flow meter is not available or a rough estimate is adequate,
you can measure flow by using a float. The float can be any buoyant object,
such as an orange or a partially filled plastic water bottle. It needs to be
heavy enough so that about an inch of it is below the water line. (Don’t
use glass or any material that may cause problems if you can’t retrieve
the float after the measurement.)
Measure off at least 50 feet along the bank of a straight section of stream.
If possible, string a rope across each end of the 50-foot length.
- Estimate the cross-sectional area of the stream at one of these ends by
using the total stream width and the average depth. (Calculate the average
depth from depths measured at 1- to 2-foot intervals.)
Total width (ft) x Average depth (ft) = area (ft2)
- Release the float at the upstream site. Using a stopwatch, record the time
it takes to reach the downstream tape. (If the float moves too fast for an
accurate measurement, measure off 75 or 100 feet instead of 50). Repeat the
measurement two more times for a total of three measurements.
- Calculate the velocity as distance traveled divided by the average amount
of time it took the float to travel the distance. If the distance roped off
is 50 feet and the orange took an average of 100 seconds to get there, the
velocity is 0.5 ft/sec.
50 ft = 0.5 ft/sec
- Correct for the surface versus mid-depth velocity by multiplying the surface
velocity by 0.85.
0.5 x 0.85 = 0.43 ft/sec
- Calculate the discharge in cubic feet per second (cfs) by multiplying velocity
(ft/sec) by the cross-sectional area (ft2) of the stream.
0.43 ft/sec x 10.73 ft2 = 4.62 cfs
- Now convert cfs to gallons per minute (gpm)
4.62 cfs x 7.4805 gal/ft3 x 60 sec/min = 2073.6 gal/min (ie. 2073.6 gpm)
Courtesy of the Washington State Dept of Ecology