Rock dam grouting.
Not easy, need to drill then grout .On a similar project a 80 ft deep rockfall filling a gorge we used a 7" drill bit 3 lines of grout holes at 2- 3 ft centers. Downstream line was first filled with 3 " rock to fill voids in large rocks, middle line was filled with 1" rock, again to fill large voids and upstream line of holes then filled with pea gravel. The intention was that the gravel and pea gravel would migrate downstream and gradually fill the voids in the 3" rock that was first dropped into the downstream line of holes.This would reduce the void sizes to as small as possible and enable grouting ,if you have some water flow.
You will need to keep topping up the various holes
as they settle.
If if you cannot slow the flow enough to grout, you need to stop the flow as much as possible by building a higher temp dam downstream to reduce the pressure differential in the first then you can grout.
Environmental considerations if fish are present should also be considered.The ph downstream should also be monitored. Grouting if ph levels are critical can be very slow to reduce the volume/hr of grout leaking into the flow to minimise ph level changes.
Alternatively if you have one or a number of definite entry channels you can use fabriform concrete
bags or mat as a plug, or just one of varois types of liners, a definite answer depends on the actual situation.
Intrusion Prepakt /marineconcrete.com