The Micro Hydropower Handbook is a technical bulletin from the US Department of Commerce. It contains information on small scale hydro-electric power. As a document from the Federal government it has some information on licensing and exemptions that other microhydro documents don’t have.
This is an older document and the laws are probably not the same now, but it gives the budding hydroelectric enthusiest a place to start their research to see if this is legal in your area.
“If it pleases the crown may I use my water on my land to light my home so my children can study to become good taxpayers”…Micro Hydropower Handbook