There is potable water from a facet at Roaring Springs, Cottonwood Campground, Phantom Ranch and Indian Garden.

Between Cottonwood Campground and Phantom Ranch the trail is next to Bright Angel Creek, but it needs to be treated.

Between Phantom and Indian Garden you will have an opportunity to treat Colorado River water, Pipe Creek water and at the top of the Devil's Corkscrew Garden Creek. A rim2rim is pretty wet compared to the Tonto.
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"In theory, theory and practice are the same. In practice, they are not."
Yogi Berra