|An adult Krider's (much darker than the one we saw)|
I've seen these a couple of other times. The previous sightings were much more like this photo, with a pink tail. However, this one was most likely a juvenile as it was starkly white with just the tiniest of dark wing tips. I excitedly described to my husband just what a rarity was this sighting.
You can learn more about these at this one of many different sites.
It passed over us, heading upriver, with barely a beat of it's wings and we were in awe in the gift we'd just been given.