I can't see much except that it is white with center of yellow that sticks up, and is in water. So I'm going to take a guess and suggest Ranunculus aquatilis, water buttercup. Perhaps you can enlarge your pic on your computer and see details to compare.

Here is a link that might be useful: Yellowstone wildflowers

If I saw that here I would guess at Common Water Crowfoot, Ranunculus aquatilis. I believe there are American species but don't know enough to say which one this is. Compare with Great water crowfoot, Ranunculus multifidus, which I find grows in Yellowstone, and see what you think.)

I think it's Ranunculus aquatilis, after deliberation. Shown is a zoomed in section. Thanks!