(in seconds) it takes a floating object to travel a known distance. A simple way to do this is to attach a known length of line (2 meters is sufficient) to a cork (see diagram below) and record the time it takes the cork to float the distance of the full length of the line in the stream current.
1. Hold the cork and attached line at the surface of the water. Position yourself so that you do not interfere with stream flow.
2. Release the cork (hold on to the other end of the line!) and start timing the float.
3. Stop timing when the cork has traveled the full distance of the attached line.
4. Record the time it takes the line to fully extend in the stream current.
5. Because velocity varies across the channel, repeat this process 5 times, sending the float down different paths at different location within the stream.
6. Calculate the average float time by dividing the sum of the timed values by the number of float trials.
7. The average surface velocity (m/sec) of the stream is simply the distance traveled (the length of the line) divided by the average float time of the 5 float trials.
8. Because stream velocity varies from surface to bottom, adjust this value to reflect the overall average velocity of the stream, by multiplying the average velocity by the correction factor 0.8 (Dunne and Leopold 1978).