If I see a pillow–reaction wave pair, does this mean that there's a souse hole between the waves?
Possibly, Ric—though this isn't always true. When fast water rushes over a barely-submerged rock, the downstream eddy often takes the form of a "hole" in the river—the water level drops noticeably. When the drop is dramatic, water flows into the hole from all sides. The result is a "souse hole", and the reflux flow at the downstream margin of the eddy will take the form of a "stopper"—or, even more evocatively, a "keeper"—a large, backward-curling standing wave that breaks UPSTREAM. This combination of hole and stopper can trap an unlucky swimmer for several anxious seconds (or even longer) before he's flushed out. Skilled boaters, of course, see holes as opportunities to play the river—they often surf the upstream faces of the stoppers. This is a trick that's best practiced in company, of course, and both boater and boat should be prepared for a wild ride.
OK. Let's go back to mid-river rocks for a minute. There'll always be an eddy below a rock, won't there?
Right, Ric. There's an "island" of relatively quiet water, or eddy, below every obstruction, large or small. In gentle rivers, an eddy may be nothing more than an all but invisible "slick," with a barely-perceptible circulation. In steep, powerful rivers, however, the water in eddies often flows forcefully upstream. These eddies aren't "quiet" at all. You can even find secondary eddies upriver of rocks located within them.
Eddy-lines are sometimes hard to spot in slow-moving rivers, but in big water, eddies often become holes, and eddy-lines are marked by dramatic "steps" in the river. You can't avoid noticing them then!