When one looks at the mass of gravel out there I really donot think you could find enough machinery, equipment or resources to take enough gravel out of there to really make much of a difference.
As well if you look at all the places they dug last year on both the Vedder and and the Fraser all the holes they dug have been filled in again.
I am not an engineer or know anything about river hydraulics but when you start loosing up gravel doesn't it start to shift around more as well as spoiling good spawning gravel.
I believe they used to take gravel right out of the Coquitlam River. They would take it off the non wetted area and during the next high water it would move to that side of the river. Next they would then take it from the other side, next high water it would reverse sides again. This I am told went on for years until the Coquitlam Fish and Game Club put pressure on and that practiced stopped but not before most of the good spawning gravel was removed. Of course then gravel mining moved to the hillsides and the runoff from that did more damage but that is another story.
It looks like this practice is going to happen on the Chilliwack with the start of gravel mining at Vedder Crossing all in the name of flood protection.