Water Flow Question

The question relates to a certain pattern or feature that is sometimes evident on the surface of slowly flowing water: WHAT IS IT CALLED?

The pattern or feature has some of the appearance of a discontinuity of the sort that seems analogous to the transition of a flow from laminar to turbulent. Here is a schematic view showing my perception of the discontinuity and the velocities upstream and downstream of the discontinuity:

And here is what I imagine to be the view in the direction A-A' (with vertical exaggeration):

I have seen this phenomenon near the locks on the C&O Canal, in Montgomery Country, Maryland, but I have not got good photos until now. Here is a photo of the flow discontinuity that I got recently - along with a view highlighting the discontinuity with a red line. (Click on picture for a larger view of the main image.)

The discontinuity seems to undulate - i.e., to oscillate slowly more in a direction normal to itself. The discontinuity is indicated in the lower view with a superposed red line:

One other view:

Bob

Uploaded: November 10, 2008, 1149 hrs