Scot - actually, a lot of water is imported into California's supply from Oregon. The reason the demarcation is so apparent is because Oregon's municipal supplies come from rain- and glacier-fed reservoirs, whereas California has to buy from many sources, including aquifers under desert and farmland where the ground water isn't so pristine. At the northern border, the rainwater is all the same no matter what side you're on, but the same can't be said for what's coming out of the taps.
~Joe