As part of the new category on accumulating micro hydro related information: This is a link to the article that Lisa Richardson wrote back in August 2004. It has some intersting points and gives the Mamquam River as an example for a high impact system. (The Mamquam River in Squamish has 2 small hydro projects—25 and 50 MWH, which is pretty big—and 7 other, smaller projects in its watershed).

A Green Thread to BC’s Rivers

Also note the (long) discussion below the article.

In my opinion, a key point in discussing micro-hydro issues is the lack of knowledge. No research has been done as to what the cumulative effect of drying out riverbeds is. A common concern within the “concerned community” is that one samll-hydro project may or may not have a substantial impact.
However, a cumulative effect cannot be understood with the limited and short term studies that hasve been conducted so far. Especially, no long term monitoring has been performed as to the effects of substantially reducing flow. Substantially reducing year round flow in a section of river may have strong impacts on the downstream sections. It seems to be a common pattern in large scale projects like this that if the knowledge is not available, we don’t worry about it.