Lakes Acting like a Mirror and a Sheet

I was recently on vacation in Banff National Park, which is in the Canadian Rockies. I did lots of hiking with a friend and I also took a lot of photos. One major attraction in the rockies are the beautiful glacial lakes, which have an emerald glow to them at times. One thing my friend and I noticed was that the lakes didn't always look emerald. In the morning, the lakes seemed to act like mirrors. Below is a photo of a lake in the morning just as we began our hike to bow falls.

Clearly, in the photo you can the reflection of the mountains, the sky, and the clouds.

Below is another photo of the same lake, taken almost at the same location, but in the afternoon as we were returning from our hike.

As you can see, while the first one shows the reflection of the mountains in the background, the second one shows more of the famous emerald color. You can't see any of the reflection.

What I'm Wondering
The difference in the photos has me wondering about why the lake acted more like a "mirror" one time and then, later, more like a "sheet". In particular, I am wondering if it had something to do with it being morning and afternoon, or if it was just a coincidence and it had something unrelated to the time-of-day. More generally, why does water act like a mirror at all? Why can you sometimes see through water, but other times you can only the color of the water (e.g., emerald)? It seems weird that water could act in so many different ways.