> To use an analogy, Let's say I am on the Amazon river (fastest river according to google). You want to detect which way I was swimming. Would you even be able to detect the marginal effects of me swimming upstream relative to the massively more impactful force from the river?

Definitely yes! If you're even a reasonably competent swimmer you should be able to outswim the Amazon and make headway upstream at most points.

I'm not sure what this says about your analogy, but I would think the measurement devices are millions of times more sensitive than needed to detect which way you were swimming.