Even if you aren’t keen on abstract photography, there is something interesting to get from this example.

Look at the left half of the image. At a shutter speed of 1/200 s the flow of water looks “frozen”, which keeps the details of the underwater vegetation.
Now compare it with the right half of the photo: that fuzzy look is the result of a maze of micro currents running at a speed high enough to avoid the “freezing” effect of that camera setting.

That is, in a distance of a few centimeters along the current you not only get a big difference in water speed, but in the turbulence of the flow as well.
It is this scenario of changing and conflicting currents what makes drag-free drifts so hard on rivers of this character.