River and the tools you already use
You do not have to start from zero. Keep both, or bring years across when you are ready. River is a daily writing app — a home for an active creative life. If years already live in another app, they can sit behind today's page. That is optional. Most people should just start writing.
River is not a notes app, a second brain, or a vault. Keep iA Writer, Ulysses, Drafts, or Obsidian if they are doing that work. See which one you open.
What we cannot hold is named, not quietly dropped. Leftovers come back as a file — original bytes, or a short stub that says why we do not have them.