If you watch the History Channel show "How the States got their Shapes", access to water was one of the key factors.
Only until we got to states west of the Mississippi river. Then the show says that railroad lines shaped the states, which is why they are are rife with box shapes and straight lines for boundaries.
When the folks living in the Mississippi flood plains have water up to their roof tops, I certainly don't think they are worried about ground water....And I don't propose that we take away all their water. Just take away the excess water that is causing flood damage.
They should be worried about ground water, not worrying about pending problems is what created our current water issues. The groundwater issues are also not along the areas which flood on along the Mississippi river, we're talking western Kansas and such places on the Great Plains.
I still do not believe that the flood waters of the Mississippi would do much good to the West, it's not that much water. There is currently a lake that is flooding in North Dakota (Devil's Lake); can we get the water from there to the deserts of America, and if we can, will it be worth the money? What happens when the flood waters are used up, as that would put us back at Square One? Building canals seems to be a stop-gap measure. Once we suck up the excess flood water, we'll still need more, and where do we get that from? Do we go with the idea of piping water from the Great Lakes out west? That's another stop-gap measure, one that robs peter to pay paul.
The floodwaters are also unpredictable. Like wind power, we don't know when, where, and how much flooding will really occur. Some bets, like the Mississippi, are safer than others for flooding but that doesn't make them guaranteed.
Another thought is that even if we do build these thousands of miles of canals, everyone along the way will want a piece of the pie; by the time the water gets from the Mississippi to Arizona, say, it'll just be a drop as all the other places will have taken their fill. I also don't see how people who are living in areas with water will be convinced that their land and tax dollars have to be used so that people who made the choice to live in a hostile environment can get bailed out.