Fox (Illinois R. trib.) - D) Yorkville: Marge Cline Whitewater Course (PnP) (IL)
Perhaps optimum playable flows.
At lower flows, this offers surfing and flat spins galore. At flows above around 1200-1500 the wave washes out. At optimum flows, the feature should allow cartwheels and loops, though at other flows it can be somewhat shallow, so attempting vertical moves may find bottom-contact (depending on where you initiate, and your boat and technique). The feature actually becomes more retentive as flows drop.