Just a side note, creek water is probably similar to our mountain rain/runoff unfiltered reservoir water that comes pre-algae'd (is that a word?). When we fill from scratch, the water turns green instantly at around 4". As I theorized last year, our source has algae already although not enough to compromise drinking quality (our tap water is good enough to bottle). Once the sun hits it, it's growing like crazy.
I'd assume creek water as similar and make sure you don't let it drop to minimum very long, especially when you add water. That'll be a trial and error sort of thing for you depending on how much water you add. Also, you will need to shock at the proper level (plus a bit maybe?) for your CYA level long enough to fully kill the algae you have now, and then you maintain FC daily according to the calculator, and wait. Patiently, while the filters do the rest of the job.