I've had several friends inquire about how we kept stuff dry on the Green River trip. Packing gear for a canoe trip is somewhat influenced by regional experience. For example, many canoeists in the Boundary Waters Canoe Area default to a device that looks like a canvas pillowcase with shoulder straps and a flap forming a lid, commonly known as a Duluth Pack -- or one of its modernized relatives. Keeps things together and easy to portage, but it's not waterproof.
If the trip involved portages, I default to an NRS "Bill's Bag" which is a 110 liter roll-top dry bag with shoulder straps.
Of course, there are other options. Every year we see canoes with black garbage bags piled high. They'll do the trick as long as they don't get any holes or tears and you aren't one of the handful each year that flip their canoe. Another option is to rent dry bags from an outfitter. Keeping the boat right side up is by far the easiest way to keep things dry. ;-)