Possible ledge/wave/hole

As the longer boulderbed rapids continues (somewhat intermittent, broken only by brief pauses), at a tight twist to the left, it appears to encounter what may be a slight ledge/wave/hole. This could be small enough to 'wash out' at good flow, but may also  (at some flows) be a fine playable wave. Light boulderbed continues maybe a half-mile downstream before becoming increasingly intermittent. These low-grade (I-II) riffles and rips continue intermittently to the confluence with the Marengo.