I have been told that here in Ontario you cannot "impede" the flow of water without a permit which is hard to get. You cannot build a dam blocking a year round or seasonal waterway and even culverts require approval on a flowing stream. You can however, divert some water from a stream to a pond with the overflow running back into the stream with no pollutants added(this includes heat) but you had better check with the MNR in each case. Beavers are not required to obtain these permits however.....