When I think of "raging river", I see the creek that runs through my property, which can move 2000 pound boulders and 80 foot tall pine trees roots and all downstream in a major flood... so the picture of a brisk stream flowing peacefully through grassland is incongruous to me
But the OP's description sounds more like my situation than later posters'. Without access to modify the headwaters, there is nothing that can be done unless there is access to large rocks, in the several hundred pound range, and lots of them. Large tree trunks in plenty might also be effective at making control structures that will not be washed away in floods.
If water rights are not involved, the idea of adding swales to conduct bankful flooding away to soak into the ground may be viable, although my experience with similar features is that the access from the streambed to swales would need to be reestablished after every flood, as the water will deposit sand or gravel or other debris in all secondary channels as the water recedes, leaving only the main channel(s) for flow.