Yikes... no reservation?! When did you say you're going? Hopefully not this weekend ...
Regardless, you might try to make a campsite reservation, I think sometimes they take them a few days in advance. The campgrounds always seem full in the summers...
If you must be near a creek, Sherwin Creek Campground is nice, obviously right on Sherwin Creek. There are aspens all over the creek, I hear they are starting to change colors right now. Should be pretty. Also try Coldwater Creek campground in the Mammoth Lakes Basin area - there are lots of other campgrounds there as well. , just
Here's a map of the campgrounds in Mammoth.
http://herronweb.com/cgnes/maps/Insets/mammoth.html
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Have a great time.