You are right Chris, you cannot take enough gravel to prevent floods. In fact, I wonder what kind of liability the Government is assuming by doing this removal. Say, for example, their actions unintentionally redirect the thalwag, which ends up flooding someone else's property, perhaps some first nation land. Who's going to knock on the Government's door the next time the river floods, to point out to them that they wasted everyone's time, money and resources for nothing.
The massive floods that occurred in Washington last month taught many people valuable lessons. Sure, existing dikes are being rebuilt, but homes are a different story. In some locations, people are not allowed to rebuild. In other locations, they can only rebuild if they lift their homes higher. There have also been cases in history where entire cities were abandoned and relocated to higher ground because of reoccurring catastrophic floods.