there is a huge wasabi farm about 20 minutes from home between me and Western Carolina University. The grow it just like it does in the streams of Japan as you describe. The conditions in Cullowhee are very conducive for cultivating it. As fins stated, it is nothing like the dyed horseradish crap you get in restaurants. It is much more flavorful and refined.

their website is here http://www.realwasabi.com/