I have a friend that works in water management out west, I asked him and he said:They get a metric butt-ton of snow every year, so I assume this is runoff. There is a river that winds through the northern(?) part of the park into Yellowstone Lake, so I can see that overflowing and causing issues.What's up with the Yellowstone rain pretty much closing the park and stranding people there. Absolutely crazy. You'd figure it would be a drought or fire that would be the issue.
"Local storms...having similar issues in the Feather River basin upstream of Oroville--one of the state hwys is cut in three spots. 2-3 inches of rain from local thunderstorms."