Hi Jasman. I have done a lot of high altitude mountain shoots over the past 20 years (Nepalese Himalayas/Japan Alps/NZ Alps/Alaska/Canadian Rockies and Coast Mountains/US Cascades/European Alps/Scandinavian Mountains/Bolivian and Peruvian Andies/ Argentinian and Chilean Patagonia) and the sky is that deep blue colour when your up in the high mountains. There are a few reasons.

The first relate to their being less light scattering when at altitude due to a thinner atmosphere (both in terms of distance to the space boundary and air density), less particulates, and also a critical one is there is much less water vapor in the air as high altitude cold air is capable of holding exponentially lower moisture content at the dew point (ie 100% humidity) than warm air. This has a similar end result to using a polarizer on the sky at sea level (the polarizer reduces the total amount of scattered light from water vapor by removing waves of a certain orientation). This is why if you use a polarizer at high altitude you often end up with a black sky (this effect was even worse in the old days with slide film as the dynamic range of the film was lower than modern cameras; however could be used for effect if done properly).

The second reason is the glacier and snow and clouds in the shot also reduce the exposure level of the whole shot as the camera tries to balance the exposure; which darkens the sky unless you actually use HDR techniques to bring back the brightness of the sky. If you think of going out in the snow; staring at white snow for a few minutes then looking at the sky; its looks black for a few seconds until your eyes adjust. If you stare at the snow for long enough you eventually go snow blind as they have trouble adjusting and everything else looks dark. This is all in keeping with @Kieranstone indicating that its a single shot. I cant see any hallowing/HDR artifacts when I look at it on a larger screen other than viewing some JPG compression artifacts. This comes from the free steemit hosting which uses a high compression ratio on the image; which I don't like, and link all my own shots to my own website.

With respect to the saturation; I agree the image is highly saturated. This is obviously the choice of the photographer and a matter of opinion as to what looks good. I would personally back the saturation off a bit; but that's just me and my style of photography is different to Kieran (if we were all the same the world would be boring). I would however not adjust the hue difference between the opaque glaciated blue of the river and the crisp dark high altitude sky as to me that is what makes the photo and thats what I love about being up in the mountains. Rob