Here are a couple of questions you might explore. Please do not try to respond to all of them and feel free to add in with your own reactions or questions.
At the beginning of the book, Thoreau points out the distinctiveness of rivers, their role in history and ultimately calling the river's current "the emblem of all progress" (13). How does the experience of river travel, its constant movement and its relation to the bank/shore affect the structure the book (whether in the way he describes what he sees or all the digressions into history, philosophy, religion, etc.)?
There's a lot of talk about Puritan history, Native Americans and myth here--all about the past. This book was also written 10 years after the trip it describes was taken. What do you think Thoreau is trying to say about the past and our relationship to it?
You might also comment on your own personal reaction to one of his digressions, whatever the topic (and while it is understandable to be frustrated, I would appreciate it if you were to comment on something other than your frustration). What do you make of it? How does it fit or not fit with your own views on religion, or wilderness, or fishing (or anything else)? How do you see it connecting to the project of the book?