That night, Tally escapes from her dorm and rides with Shay outside of the city.
(Escaping from the dorm is pretty easy, almost like the city planned to have uglies play their tricks. Hmmm.)
The hoverboards only fly over magnetic material, like metal or the metal deposits in the river. So Tally and Shay can ride up part of the river.
By the city, the river is smooth and calm; but upstream, the river turns wild, with white water and rapids. It's almost like the setting (calm by the city, wild outside the city) is trying to tell us something.
Being out in the woods at night is much better than breaking into New Pretty Town—you don't have all those drunk people to deal with, for one thing.
But then Shay and Tally have to walk a bunch because there's no metal for their hoverboards to hover over. (Do not say that five times fast.) And then they reach… the next chapter.