This picture was taken at Horsetail Fall in Yosemite National Park. Apparently this happens every year but it's hard to catch a picture at just the right moment. Photographer Sangeeta Dey explained this on a post about her own picture on Facebook:
Every year for two weeks in February, the sun sets at a certain angle and illuminates the waterfall in luminescent orange and red, making it look like a fluid fire. I’ve met photographers who said that they have been coming for 11 years only to see this happen 2 or 3 times.
via Sangeeta Dey