What can you do? Wash your car on your lawn rather than on the pavement will make a big difference in how much soapy water would enter a nearby stream or Lake Superior.
A friend of mine mentioned a product that she came across promoting a way to wash your car without water. I looked on the internet and there are several products available.
Eco Touch is one of the waterless car wash products. You basically spray it on and wipe it off, like a window spray.
In this case you are using essentially no water (conserving water) and not creating polluted runoff.