Like momonthego, I toyed with the theory MC was transported by boat. After visiting the boat launch at Orondo St. This week, I'm not ready to put that theory to rest.
I'd asked if there was a place at the shore near the boat launch where MC and a perp could be out of sight... I believe there is. Just south (I think it's south) of the launch there is a rocky "path" that leads down to the water. It is well-concealed by trees from the public area, and it leads to a small, rocky "beach" right along the river.
I wonder if it would be secluded enough, in February, for the act to have taken place there. She may have been left there, in the hidden rocky area, for a time until the perp returned with a boat and launched it at the launch.
Then, he could have tooled around the river a bit - just a guy doing some fishing or whatever - until he pulled up close enough to the shore to get out and put her in the boat.
Here's where my theory falls apart - it would have to be a big enough boat to have a "below deck" area to hide her, KWIM?
Anyway, then he could have pulled the boat out of the water, trailered it, and driven to one of the launches on the other side of the dam.
I don't have much faith in this theory, but it's one possible way a boat could have been used. I had my toddlers with me at the launch, so I didn't get a chance to go down and explore the "beach" area.