Actually you are close, I believe the longest portage; grand portage, is closer to 8.5 miles, not bad for crossing 6000 miles by rivers and lakes. Gary and Joanie McGuffin made this trip and wrote a great book about it.

http://www.adventurers.org/RiversRun1.html

: Well, anywhere in Canada. I undrestand that you can cross almost the entire
: country by water. the total amount of portages is supposed to be well
: under 100 miles. ( I think I heard or read it was 63 or 73 miles), and the
: longest one was under 5 miles. That's not bad for crossing a continent!
: Anyone with more accurate data on this, please let me know.