"Quiet on the creek" doesn't reference any real body of water. It's the newest way Gen Alpha kids tell each other — or occasionally a teacher — to pipe down, usually said loudly, which is part of the joke.
The phrase seems to work the same way a lot of this fall's slang does: take a random, oddly specific phrase, strip away any literal meaning, and let the sound of it carry the joke. There's no deep backstory — the appeal is that it sounds like it should mean something serious (like a film set direction) while actually just meaning "shush."
It's been spreading through classrooms alongside other new back-to-school terms like "larper," part of a wave of phrases teachers have been documenting as this year's fastest-moving slang. The pairing makes sense — both are absurdist, both spread through repetition, and both work best shouted rather than said quietly (ironically, in this case).
Like most nonsense-phrase memes, it probably won't survive the school year — but for now, it's doing its job.
Check the decoder for "quiet on the creek" alongside "larper" if you're trying to keep up with what's landing in the hallways.