I have built 3 ponds over the last 5 years and have learned some lessons the hard way. To answer your question, I like siphon systems but it can be tricky to figure out the size you need for storm events. I imagine a reputable pond builder can help with sizing by looking at the drainage basin, soil types, etc.

I second the warning about damming a flowing stream. There used to be US Fish & Wildlife map online that showed federally recognized wetlands and "waters of the u.s.". It would be risky to disturb those areas without approval.

Best of luck to you!

-Jason