Yup, squats are also good.  You'll appreciate them when you have to negotiate ledges.  

However, a Stairmaster is the best simulation I've found for portaging a distance under a load.

Squats=power.  Stairmaster=endurance.

I do a stairmaster once a week for 3-4 months prior to a canoe trip.  The rest of the week, I run or use one of the other various hamster wheels at the club.