>For it to look correct requires simulating everything about it, everything feeding into it, everything it affects, from the moment I stopped looking at it until I looked at it again.
No, it only needs to at the two points you take inputs provide answers that satisfy your understanding on what the inputs should be, and your understanding is formed by other inputs you've been fed, making the entire thing much easier. Satisfying you is probably much easier than satisfying someone who really studies rivers.
Neither you nor an expert needs simulating it much at all while you're not looking at it. And I doubt there's any case to simulate to any atomic or subatomic level. The gravitational pull of Neptune can likely be ignored. Neutrinos that may scatter from Antartic ice may hit it, but can probably ignored. And on and on. You're inspection is no where detailed enough to check if these things were taken into account for a decade while you were away. All of this detail is irrelevant.
It may well be possible to do a good job with a very rough simulation of water and time, all of which could be computed on demand.
So really all that needs stored is enough state to simulate in case you look at it again, and no work needs done on it between those looks. And this is a trivial amount of storage and computation to satisfy your two causal glances when compared to needing to simulate an entire universe down to all 10^120 subatomic particles.
No, it only needs to at the two points you take inputs provide answers that satisfy your understanding on what the inputs should be, and your understanding is formed by other inputs you've been fed, making the entire thing much easier. Satisfying you is probably much easier than satisfying someone who really studies rivers.
Neither you nor an expert needs simulating it much at all while you're not looking at it. And I doubt there's any case to simulate to any atomic or subatomic level. The gravitational pull of Neptune can likely be ignored. Neutrinos that may scatter from Antartic ice may hit it, but can probably ignored. And on and on. You're inspection is no where detailed enough to check if these things were taken into account for a decade while you were away. All of this detail is irrelevant.
It may well be possible to do a good job with a very rough simulation of water and time, all of which could be computed on demand.
So really all that needs stored is enough state to simulate in case you look at it again, and no work needs done on it between those looks. And this is a trivial amount of storage and computation to satisfy your two causal glances when compared to needing to simulate an entire universe down to all 10^120 subatomic particles.