Years ago, a guy by the name of Tim Matson wrote a book about what you are thinking of doing. He calls them "digger" ponds. His book
Earth Ponds is available though the Pond Boss store. It has some extremely good advice on building the "digger dams" that carve out the area below them.
One minor point is that it probably is a fine line in defining what constitutes blocking a stream. Should it become an issue, at least the dams described in the book are easy to remove. They are also made from local tree trunks, usually oak.
Good luck,
Ken
