Here is a link to one of the bigger whitewater park designers. They have a list of parks they have done. Look at the pictures.
Dams | REP
Here is a corner pocket video taken several days before washout, 4200cfs, if anyone knows the kayaker, send him the link.
Another alternative to grout is precast locking forms, like big legos. If that could work it would cost a lot less than grout since the river would not need to be diverted. Precast cement is cast into forms at the plant and then trucked in, like roadway barriors.