Just because a one word modifies another doesn’t mean they can't be joined into one: dragonfly, airport, parrothead, they're called compound words. All the above are listed in dictionary.com. Once general use and acceptance of a word becomes commonplace (another compound word), the dictionary publishers consider it for addition. Case in point: if you're of a certain age, you were taught that ain't ain't a word...but it is now.
My point, if we all start, or continue, using stormwater (one word), eventually it will become universally accepted and added to Webster, and this whole thread will become moot. Until then, with the lack of an engineering dictionary equivalent to a medical dictionary, the Civil Engineering Reference Manual for the PE Exam uses one word "stormwater".
And, glenncrowley, why are you yelling?