That's really cool, most of the J- dams or Wing dams or whatever you call them that I've seen constructed recently are very carefully constructed with a low point where the main flow is concentrated to one area instead of evenly flowing over the top.
It is more naturally flowing and still allows passage, Muddy Creek and Fishing Creek (York Co.) are two examples I've seen this.
The dam like structures are actually U shaped and direct the flow to a central point which creates a deep hole and channel below the structure for fish to hide and also allows them to still be able to move up or down stream.