No US visa is needed. Valid I94(the lower part of your I797) is valid (indepent of visa) at US-Canada border. You shouldn't give it back when you leaving US with the plan to coming back.But at US-canada border, no one will take it.
-youbet(You Bet);
2002-7-1{45}(#611032@0)
Seems the above 2 replies are conflict?
-aiattendant(soho.aimoo.com);
2002-7-1(#611167@0)