The zipper runs across the back horizontally. It also has a flap that runs across the length of the zipper preventing water from entering while it is open.

I've bought a number of other bike related items from MEC. They are not cheap, but they are well made and designed with cyclists in mind. They also have great waterproof booties you can put around your shoes to keep your feet dry. No web site really lets you explore like being ther but here it is.
http://www.mec.ca/Products/product_m...lder_id=668507