Jim,

A 45 pound pack might be lightweight. Going into the Grand Canyon for six nights. Food at 2 pounds per day = 12 pounds. We will not reach a reliable water source until the evening of the second day so 7 quarts of water = 14 pounds. You need a robust pack to carry that weight so 15 pound base weight.

It is possible to sherpa and sometimes carrying a heavy pack is OK.
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"In theory, theory and practice are the same. In practice, they are not."
Yogi Berra