River Storage Discussions

[QuintillusCFC]

Splitting off from #239 , where we discussed options for storing rivers. The high-level summary is that how we're doing it now takes a lot of space, and it would be nice to be more space-efficient.

The goal of this discussion is to ensure what we already discussed there doesn't fall into a void after the PR is merged, and to eventually decide on a longer-term solution. Priority is probably not very high since save file size is not our biggest concern at the moment.