This theory most certainly does not explain why every river is concreted, completely destroying any semblance of natural beauty. Take the river flowing through Kyoto. It's an enormous, concrete culvert disfiguring the town. It has the tiniest trickle of water 'flowing' through it.
I used to live in sleepy Gunma near a sleepy river that has never flooded in living memory. Yet the banks of this rural river were concreted a few decades ago, and what was a beautiful resource for population is now and ugly, stagnant monstrosity.
Read 'Dogs and Demons', or just have a quick look at this guy's blog http://joshnjaxnjapan.blogspot.co.uk/2010/03/concrete-rivers-of-japan.html
... and this article demonstrates how concreting rivers has *increased* flow and flood risk (the original motivation to concrete them having been simply for the sake of it.)
And besides, even if *some* rivers do have to be controlled, there are other ways to do it other than smothering them with concrete, as the Jubilee River demonstrates.
PLUS concreting the hundreds of small streams that criss cross the countryside, producing deep culverts, has killed dozens of small children over the last few years who fall in and can't climb out. It's madness, and it's madness to try to defend the Japanese frenzy for covering their country in concrete.