The 6 tenths rule.

That's right. The average velocity for this hypothetical stream is 0.29 m/sec. (Your value may have been slightly different.) This value is for a very rapidly flowing stream.

Rather than make a number of measurements at various depths to estimate the average velocity, let's use the graph to figure out approximately what depth in a stream would have the "average" velocity.

This is a very important concept. The graph can tell us that there is a certain depth at which we could make just one measurement to estimate a stream's average velocity.