Okay so when Tris says “the lake” dried up and is just a marsh now, she means all of Lake Michigan. That’s some apocalypse, whatever the heck it was. FYI yes, even the narrow direction across Lake Michigan is farther than you can see when you’re standing on the shore.
How did they set up the farms?
Only a small part of IRL Chicago is inhabited in Divergent, but it doesn’t seem like any of it was really destroyed. Tris travels through areas of run-down, abandoned buildings, but almost all of them seem to still be standing, and some of them are still usable. And there’s a bit (when she’s on her way to Navy Pier for the capture-the-flag game, I think?) where she mentions that the buildings around there are all neatly locked and boarded up, like the old owners had plenty of time to prepare to leave, and that in some areas of the city that’s not true.
So I’m wondering where exactly the wall is, the border between the city and the farm area. To get to a farm from downtown Chicago IRL, you have to go pretty far! Either the wall is really damn long, and encloses more abandoned areas than inhabited ones, or they would have had to demolish a lot of the old suburbs to clear land for farming.
There’s an interesting sort of disconnect between the level of technology that exists in the city, and the technology the average faction member has access to. Erudite is building solar-powered cars, but most people don’t have a car unless their work requires it. Computers exist– Dauntless uses them to monitor security cameras, and Tris sees people studying on computers in the Erudite library, but nobody seems to have their own computer, or even a phone. There’s no sense that any of this technology is old or wearing out or hard to repair, but when Abnegation organized an effort to repave the streets, they ran out of materials before they finished all the inhabited areas. Where does the city get things besides food? There has to be some kind of manufacturing going on, and they have to have raw materials for it, but where does that happen?
On a less meta note, I really like how much this book is about Tris figuring out who she is and what she’s capable of. I almost wish there was more of that.