MP, those links were very helpful.
Rip, they say that there are both American and Gizzard Shad in the Merrimack. Those flies look like what I'm seeing in the local Fly Shops. I guess since the fish are not, generally, feeding during their spawning runs, that you're trying to annoy/irritate them into a territorial striking behavior.
Yes, they stock Atlantics above the dam in Lawrence; actually quite a distance above it. When the conversation turned to Salmon, there were comments both about fish coming "over the dam" and of them coming in from the ocean. I have no first-hand knowledge of the accuracy of the reports, but everyone seemed excited to be able to talk about Atlantics at all in the Merrimack.
From where I live, I can see Rocks Village Bridge and everyone says that's the place to fish for Shad, so I guess the structure part plays a pretty big role in where they locate themselves during the run.
The Merrimack is an interesting story for me. I grew up on the river; but in the '50's-70's it was so polluted by the effluents from the mills in Nashua, Lowell, Lawrence and Haverhill, that no one fished it seriously. I think it was classified as a Class H river at that time; which I always considered put it on a par with the Ohio. Now it's a Class B river; so lots of progress in the right direction in 30 years time. And, apparently, the fish have noticed.
Thanks for the referral on the book; I have it on order.