What does Bryson find disappointing about the Mississippi River in the nonfiction book, The Lost Continent: Travels in Small-town America?

When Bryson first visits the Mississippi, he observes it as disappointing: not because the river itself is disappointing, but because his day is disappointing, and it, not being fantastic in his eyes, is the only mental stimulation he had that day. The second time he visits, he compares his adoration of it to his adult experience and finds it dull in comparison.