Geologic Work of Streams

Stream flow and work

Figure 18.30 shows the relationship between particle size and minimum flow velocity for the three types of geologic work performed by water. Examine the minimum flow velocity for erosion. Note that it is much more difficult to erode materials from the channel than it is to transport or deposit them. For the finest particles like clay, a very high velocity of flow is required to dislodge them from the bed of the channel due to their strong molecular bonding. Small particle don't protrude above the bed of the channel so moving water slips over them more easily than larger particle that stand above the bed more. At the other end of the spectrum, the largest particle require high flow velocities to erode them.

Figure 18.30 Critical velocity (cm/sec) for
erosion, transportation and deposition

Examine the area of the graph labeled "Transportation". Note that once clays have been eroded they can be transported over a wide range of velocities, even very slow ones before being deposited. The area of the graph for transportation narrows as you move to larger particle sizes. Larger sized material is harder to transport due to their weight.

Take a look at the area of the graph labeled " Deposition". Notice deposition doesn't extend over to the very smallest size on the left. The very smallest particles are easily transported even under low flow velocity and will not settle out. The minimum velocity for deposition ( the line that separates transportation and deposition) climbs steadily to the right. This indicates that as particle size and weight increases, it is more difficult to transport material and deposition will occur with a slight drop in stream velocity.

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For Citation: Ritter, Michael E. The Physical Environment: an Introduction to Physical Geography.
Date visited. https://www.earthonlinemedia.com/ebooks/tpe_3e/title_page.html

Michael Ritter (tpeauthor@mac.com)
Last revised 6/5/12