The 'Fire Falls' in Yosemite National Park are a rum old thing. It's all to do with the angle of the sun hitting the water, the height of the sun, the water not being frozen and the clemency of the weather. Apparently this can only happen in December and/or January. Given the frigidity of the winter in Yosemite, there is no assurance that this sight will be seen whatsoever.
Bloody impressive, eh?