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'.

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.