The area is a place where opposing currents meet. I'm sure mariners have a name for the phenomenon, but I don't know what it is. As two currents meet debris and foam are trapped along a line where one current dives beneath, or is forced to run alongside, another. The linear features persist until tide changes or wind changes no longer force the currents to strongly oppose each other.

Below is a phenomenon that matches what I an trying to describe. Water is flowing in from the sides of the man-made rapids, as well as flowing down-stream. Where the currents meet, the foam and debris caused by turbulence forms a linear pattern reaching off downstream.