The emergency spill way is in case of a large rain or melt event that brings the water level up quickly. The spill way is used to ensure that the dam is not over topped, which is usually catastrophic. As such, the spill way is usually placed at or near the maximum safe free board (distance from the top of the dam down to the water level). What this all means is that the spill way can only be used when the water level in the reservoir is allowed to get to high and possibly dangerous levels. The article stated the the water level has already been decreased by 10 meters which means that the spill way is going to remain dry for a long while.
Next time you are near a dam look around and you will find that most have three paths for the water to leave the reservoir. The first is through the power generating turbines, second is the sluice gates which control water level, and third is the emergency spill way which is designed to fend off catastrophic failure.