Uhm..... where does the pollution come from? If you are just using water without soap or any other car cleaner, it is just water and dirt. If you park your vehicle on the grass, or wherever, you are watering the lawn while you clean. In my case it would be the gravel alley outside the garage.
Oil, anti-freeze, and other contaminants.
As you may have heard, the "solution to pollution is dilution", which is true in many cases. Cleaning the vehicle over gravel is just fine. By the time the dirty vehicle water reaches flowing water, it should very clean.
The trick is to avoid the run-off into the storm sewer system where pollutants accumulate and then 'shock' streams or rivers during a heavy rain.
I do not want to exaggerate the importance of this. Compared to the on-going damage that farmers and ranchers do to salmon-bearing watersheds in British Columbia, this is likely pretty trivial. Moreover, there is not much any of us can do about all the oil and other contaminants that runs off public highways and roads during an early wet-season rainstorm other than keeping our vehicles in great shape -- something that everybody on this forum is already doing.