Hi! I've been involved in the Twin Cities' shanty sing and Morris dance/folk dance communities since I was a child, and Michael is a friend of mine. We have danced and sung together many times, and I was lucky enough to be in the room the first time this song was sung for a general audience of Morris dancers. Just wanted to create some addendums to this post for posterity.
The Michael Shewmaker you've identified as the author of "Penumbra" is NOT the same Michael as ours here in the Twin Cities. Also, the reference to the lock and dam #1 is a direct shout-out to the industrial significance of the Mississippi waterway to the Twin Cities; historically, it was used mainly for the transport of lumber (logs were floated from northern MN down the river, milled in the TC, and then transported further downstream), but has also seen barges of iron ore, flour and other milled goods, railroad supply, beer, trash to be incinerated, and, contemporarily, it hosts a hydroelectric dam at Saint Anthony Falls. The lock and dams allow barges to navigate the length of the waterway safely, by creating a kind of water elevator for the boats at the site of waterfalls. Lock and dam #1 is located between southern Minneapolis and the western bank of Saint Paul.
The title of the song, "Confluence," refers to the convergence of the Mississippi and Minnesota rivers, and, based on the context in the song, probably also refers to the confluence of Minnehaha Creek (a site much beloved by Twin Cities Morris dancers) with the Mississippi as well, just a mile upstream.
The land surrounding this confluence -- cherished as it is today by folk dancers, fishers, birdwatchers, weekend warriors, and recreationalists of all ages and walks of life -- is also a sacred site to the Dakota native peoples, and in that tongue is known as Bdote.