This isn't so much grinding the river, rather, they are dredging the river. Is a shame so many crushers operate on the river and they don't mine the river properly. This is similar to the trench in Golden Grove. The depth they are digging the river path is the more important thing happening here. River needs to run below surface. When the storm comes it will help to cut that trench more. Rivers and canals need dredging.
People are living around the area from long ago, so they will have to do this againg and againg; I think the salution for this area is to build a concrete wall along both side, and then pack stone and gravel along the wall tapered down towords the center of the river. That should solve the problem for good...
