Satoshi, New England sounds similar to Japan in terms of the number of dams on small streams. Most were built to power saw mills and gristmills over a hundred years ago, or to divert water for farm ponds and agriculture. During the industrial revolution many more were built; it's amazing how many small factories dotted the country side wherever good water power could be found. I've often been surprised to find old mills on small rivers and streams in the middle of nowhere. A lot of these dams are starting to come down as they've become structurally unsound over the years.

Here in New England all the dam construction which took place on major rivers has been disastrous for migratory fish like Atlantic Salmon, Sturgeon, and herring. Dam construction on smaller rivers and streams has played a major role in habitat loss for brook trout.

In Southern New Hampshire where I grew up a dam was removed on a small river (the Souhegan in the town of Merrimack), opening up 14 miles of river to the much larger Merrimack River, probably for the first time in at least 150 years. I don't know if any effects have been felt yet, but there have been additional proposals to remove two more similar dams on the Souhegan in Milford NH, opening up another 6-7 miles of river. It's late now so I'll look up some info on the dam removal projects on the Souhegan later.

Edit: couldn't resist googling the dam removal in Merrimack - http://www.nmfs.noaa.gov/habitat/restor ... amcam.html