Wow... what an intense thread! After reading it all, two main thoughts come to mind. (I didn't necromance it, so don't blame me.
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First, I've spent quite a bit of time exploring in the Rockies. I especially like stomping around mountain meadows and admiring Nature's little engineers, the beaver. It's pretty clear why there are meadows. If we didn't have beaver, they would just be steep, rocky, roaring runoff channels in the spring, and dry come summer. I have to think that's where people got the idea to build gabions. It's also interesting to note, in late summer, just how far the lush green areas extend past the marges of beaver ponds.
Second, I'd like to put the upstream/downstream hydrology thing into a continental context. Let's take the Mississippi River, from its outflow into the Gulf, all the way to the Continental Divide. Its catchment is about 2/3 the land area of the lower 48 states. (I don't know for certain, just looking at a map and taking a guess.)
So far as we know, the Mississippi has always had its floods, but we can hazard a guess that they weren't always so bad. The floods used to deposit silt on the Delta. Now the Delta is washing away. Why? Obviously more water is flowing down the river, under less control. But why?
Engineers. Both civil, and the Army Corps type. The goal of civil engineers, for decades, has been drainage, drainage, and more drainage. Make that water run off downstream as fast as you can. Mix that with the hundreds of thousands of acres we cover with more concrete, asphalt, and other impermeable surfaces (haha, like conventional cropland), and we have a recipe for disaster. Now see how important those little beaver dams all over the mountains are?
The problem with flooding on the Mississippi... and of course the Ohio and Missouri rivers too... is not torrential rains. It's runoff... and the bulldozers that have been speeding up the runoff for 75 years. They burn and bulldoze vegetation along the channels and streams. They poison cattails. They dredge the rivers. All in a vain effort to speed up the water... which is exactly what's causing the floods they're trying to prevent!
So, to get at Emerson White's assertion --
yes indeed, there are consequences downstream.
The solution, I think, does not lie in multi-billion-dollar FEMA projects and million-man-hour government engineering think-tanks. We don't have to slow the Mississippi itself, directly. We don't have to touch the Platte or even smaller tributaries. We just have to slow the runoff where it starts. Think beaver dams. One beaver dam might not do much, but a thousand of them will be noticed in New Orleans. Millions of gabions and check dams and farm ponds will slow the river, save the towns, bring back the silt deposit. That's just for starters. Imagine millions of acres in perennial culture, absorbing the water it sheds now, rejecting the plow.
Mr White asserts that slowing water upstream will deprive someone downstream of water. Perhaps for one season, or a few if there's drought. So it's true, temporarily. But all the water inevitably, thanks to gravity, ends up somewhere downhill of where it fell. Whether it pops out in springs, or recharges the Ogallala aquifer, or overflows your check dam in a rain... it's going downhill. No one will be deprived of anything but flooding and FEMA trailers.
The one certain inevitable result of increasing upstream storage and infiltration, and slowing the downhill flow, checking the runoff -- is that there will be more water stored on the land, for use by everyone. Once the water hits the Gulf, we can't use it. Think of holding it back as charging one ginormous battery.
Think big, Mr White. We can no longer afford not to.
