DO NOT TRY THIS! I got some misleading information online, believed that there'd be "frequent portages" but really the river downstream from Bangor is one continuous portage. About halfway to South Haven (couldn't say exactly where) there's obviously more effort to clear the worst deadfalls, but putting in at Bangor would be a huge mistake unless you want practice portaging. The aptly-named Black River's murky water means you won't see snags until you're riding onto them; its banks are steep and slippery, and the river bottom is like glue. Terrible.
None. Surprisingly wild river once you leave Bangor.
Lion's Club Park is a few blocks west of the center of Bangor. Take M-43 to Hamilton Street, turn north and look for a park to the right just before the bridge. Then turn around and go home (or find a place farther downstream to start).
I am posting this to warn others. There's a group that boosts this "water trail" that I feel misleads people.