Upon a H1B visa stamp, you will be issued a new I-94 everytime when you go across the land border. On the back of the new I-94, it states that when you leave US by acrossing the land border, you have to give back the I-94 to Canadian Official.
As to getting a new I-94, you'd better ready to show your 797. So bring with you just in case, although you don't have to (they will check out all your info from their computer database).