The run starts in Devils Postpile
National Monument. You can take a shuttle from Mammoth to get there. I would recommend to stop at the meadow along the way if wildflowers are blooming, as it is spectacular.
From the trailhead, you follow the San Joaquin river in a beautiful pine forest. The Devils Postpile
is the first landmark you'll see, where columnar basalt formations jut up about 75 feet from the ground where you're standing. If you want to make a side trip, there is a short loop to the top of the postpile, where glaciers have smoothed it out.
The trail goes into a old burn zone, that has recovered magnificently, with wildflowers and grasses dominating, with new, young trees growing back in. Soon you arrive at Rainbow falls, a 101 foot waterfall on the San Joaquin, over a very slow eroding basalt. Unlike other river falls in the Sierra (minus Yosemite), it is a free fall. There are plenty of namesake rainbows here.
As you continue along the trail, the grasses and wildflowers are dominant on one side, while the other is wooded. When you arrive at the Lower falls, there is a great swimming hole. The lower falls is more of a river fall, wide and diagonal, but the amount of water makes is a special place to swim. Retrace your steps to return.
This trail has an astounding amount of wildlife along it. We saw bears, lots of deer, and woodcutter ants (while they were building their sawdust pile from in a log). The flora consists of a pine forest environment, and a riparian riverside vegetation along the first portion of the trail. As you get farther along, there is a burn zone, with lots of wildflowers in spring, and fall when we were there.