Hi Don
They are living in all parts ot the world(climate) numerous different species, around here (Norway) you find them in high mountain creeks, streams and lakes all the way up north, in typical good arctic char habitat.
So they would definetly live in Ontario as well
http://www.waterwereld.nu/vlokreefteng.html
http://somethingscrawlinginmyhair.com/2008/04/12/freshwater-amphipod-gammarus/
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92812&page=1

cheers

PAUL