| Quote #1
Lina knew about the generator, of course. In some mysterious way, it turned the running of the river into power for the city. (1.67)
Everyone in Ember knows where power comes from… kindasorta. Lina's never seen the river (at least not in the beginning of the book), she just knows that it's water that continually flows and somehow is connected to electricity for the city. Don't get us wrong, a lot of us in the modern era are ignorant about exactly how our gadgets work—but we're not as in the dark as Ember's citizens are (pardon the pun).
| Quote #2
The mayor's office was in the Gathering Hall […] And there were offices for the guards who enforced the laws of Ember, now and then putting pickpockets or people who got in fights into the Prison Room, a small one-story structure with a sloping roof that jutted out from one side of the building. (2.62)
It seems like Ember is a pretty peaceful place overall, but they still need guards in order to maintain the current power structure: mayor at the top, everyone else at the bottom, ideally not acting up or fighting. The more we see of the mayor's guards, the more we get the sense that they, like the mayor, are a bit too happy being in charge, and that they don't mind throwing their power around.
| Quote #3
Wide as the widest street in Ember, churning and dipping and swirling, the river roared past, its turbulent surface like black, liquid glass scattered with flecks of light. Doon had never seen anything that moved so fast, and he had never heard such a thunderous, heart-stopping roar. (3.10)
Being face-to-face with such a powerful force of nature the first time around must be overwhelming, especially for an Ember kid who'd never been exposed to anything like it before. They don't have weather in Ember, clouds or rain or lightning, nothing that could prepare you for a sight like this.