Rainbow Falls is a short hike from the Rainbow Falls stop on the Mammoth Shuttle or a longer 5 mile hike from the Devils Postpile stop on the Mammoth Shuttle. Since we never take the easy route, we hiked the longer 5 mile trip. This was also due to the fact that neither of us read the shuttle guide and did not know that there was a separate Rainbow Falls trailhead stop. But it was a beautiful late August day in Mammoth and we didn't mind the extra few miles.
The walk is not overly strenuous and passes some areas nearly barren after fires which swept through several years back, in 1992 and dubbed the Rainbow Fire, or so I read, and, as we all know, everything you read on the internet is completely accurate. You'll find plenty of people hiking the labrynth of interconnected trails in the area.
The falls are hardly impressive in stature, in a state where nearby Yosemite's falls tower plenty higher. But you don't have the Yosemite Valley crowds here either and that is a definite plus. On a clearer day, which you'll usually find in Mammoth, you'll see the rainbow prism reflecting off the 100 foot falls. We didn't have such a clear day so we just enjoyed the spray of mist from the falls before heading back.
It's an easy 2.5 mile hike from Ranger Station, past Devils Postpile and onto Rainbow Falls. Shuttle bus from Mammoth Mountain Lodge takes you to Ranger Station. Bring a lunch and eat at the Falls, or a little further down at Lower Rainbow Falls. Hike takes you past trees burned in forest fire. See how the forest is regrowing itself.
The name says it all. In summer, you hike to it there is always a rainbow formed by the mist of the falls--just beautiful and refreshing after the hike. You can also see the Devils Postpile, which is quite amazing.