Water Velocity & Transported Particle Size

This graph shows the direct relationship between the velocity of a stream and the size of the particles it can transport. The faster the stream flows the larger the particles it can move. On the left is particle diameter, the size in centimeters. The colors show the range of sizes that are classified as clay, silt, sand, etc. all the way up to boulders. These size ranges correspond to the values found on page 7 for the Grain Size of land-derived sedimentary rocks.

Keep this in mind: If a stream is moving fast enough to carry cobbles, it is also carrying everything smaller than cobbles. As it slows down the largest particles drop to the bottom (settle out) first followed by smaller and smaller particles. This is called 'sorting'.

The red lines drawn on the graph (right) show how it can be used to answer typical regents questions.

What is the largest particle that can be carried by a stream moving at 5 cm/s? Follow the red line up from 5 cm/s and then, at the black line, go left. The answer is 0.1 cm. or perhaps the answer might be sand (depending on the choices).

The same question might be asked this way: How fast must a stream be moving in order to carry a particle with a diameter of 0.1 cm? Follow the red line from 0.1 to the black line and then down. The stream must be moving at least 5 cm/s.

       You should also pay close attention to the scales, both vertical (Y axis - particle diameter) and horizontal (X axis - stream velocity). First notice that the increments, the little marks, are not equally spaced. Next, and this can cause problems if you're not careful, the amount of increase from one mark to the next changes!
      Look at the blue arrows on the bottom. The increase from one to the next is 0.1 (0.1, 0.2, 0.3 etc). Yet in the next 'zone' (red arrows) the numbers jump by 1 (1, 2, 3 etc). Further to the right (the black arrows) the increase is by tens (10, 20, 30 etc). The same thing happens when you look at the Y axis (particle diameter). The increase jumps from 0.1 to 1, and finally to 10.

 So be aware and read carefully!

Back to ESRT Index