An eddy line is the seam between a river’s faster, main jet of current and the slower current of an eddy. Crossing a placid eddy line gives pause on quiet waters—time, perhaps, to leave the mainstream and admire a pale ellipse of sand draped along the bank. In formidable white water such as the Colorado River through the Grand Canyon, the shear zone between a screaming downstream current and the upstream spin of an eddy may turn the eddy line into an eddy fence. The two countercurrents rip along each other, separated by a purgatory fault that can surge several feet high. A boat on the eddy side of the fence may be looking “uphill” into the main ﬂow, perhaps in the precarious state of either being eddied out or made an eddy prisoner. Eddy hydraulics are tactile reminders that ﬂowing water has shape and form, that river topography is terrain in motion.
At a likely looking pool in the Roanoke River of Virginia, shaded under a canopy of silver maples so thick that the light beneath them looked green, my brother-in-law, Steve Schmitz, cast a spinner out onto the eddy line.
— Tim Palmer, The Heart of America