That's Yosemite's Horsetail Fall, which flows over the famous granite cliff El Capitan. And even though it looks like a burning ribbon of lava, actually, it's just water.
The reason the water looks like it's glowing is because each year the setting sun lines up just perfectly, bathing the falls in gold and red light. The resulting phenomenon is known as a "firefall."
The firefall can happen for a couple of weeks in February each year, but only if it's clear. There also has to be enough snow and runoff to feed the the waterfall, which is more than 1,500 tall.
If the conditions are just right, the firefall appears for a few minutes around sunset. It's sort of like Manhattanhenge in that it requires everything to line up just right, with an amazing result.
