At Rainbow Falls the Middle Fork of the San Joaquin River drops 31
meters (101 feet) over an andesite and rhyodacite cliff. It is thought
that after the last glacier melted, the river flowed downstream from
Devils Postpile in channels about 457 meters (1,500 feet) west of its
present course. Flowing in these older channels, it cut through the
rhyodacite lava down to granite, leaving a cliff of rhyodacite for its
eastern bank. Then, some distance upstream, the waters were diverted
eastward. The river left its bed to follow its present path until it
returned to the old channel, by cascading down the cliff it had earlier
cut. Thus Rainbow Falls was formed. A stairway and short trail lead to
the bottom of the falls, where numerous flowers and grasses form an
enchanting garden. Cars may be driven to within 1.6 kilometers (1 mile)
of the falls via the road to Reds Meadow.