How to Calculate River Velocity

Written by william hirsch | 13/05/2017
How to Calculate River Velocity
Calculate the velocity of a river by timing a floating object. (rafting,whitewater,river,colorado river,eagle,colo image by Earl Robbins from

The velocity of the flowing waters of a nearby river depends on local rainfall amounts and elevation. Scientists and engineers define velocity as how fast an object's position changes as time passes. An object moving in a straight line, in a river for example, has velocity equal to the distance it moves divided by the time of travel. A common unit for velocity is feet per second. By using a few tools you can calculate the velocity of a river.

Measure a straight distance along the bank of the river in feet. Make the measurement as close to parallel to the river as possible. Choose a part of the river that is free of turbulence. For example, consider a length of 20.0 feet.

Position a piece of paper at each end of the measured distance. The papers will be used as reference points for observing a floating object. Call the more upstream paper "A" and the more downstream paper "B."

Drop a chunk of styrofoam in the river approximately 15.0 feet upstream from paper "A," or as far away as you can, whichever length is shorter.

Start the stopwatch when the piece of styrofoam reaches paper "A." Stop the watch when the styrofoam gets to paper "B." As an illustration the time might be 7.0 seconds.

Divide the distance between paper "A" and "B" by the time to obtain the velocity of the river in feet per second. Completing the sample exercise, the result is 20.0 feet divided by 7.0 seconds, or a velocity of 2.9 feet per second.


Make sure to remove the styrofoam chunk from the river after completing the experiment.

Tips and warnings

  • Make sure to remove the styrofoam chunk from the river after completing the experiment.

Things you need

  • Tape measure
  • Paper
  • Styrofoam chunk
  • Stopwatch

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