There are some very beautiful lakes throughout the Rockies.
I'm not sure if it is due to a lack of storms... I can remember thunder, lightening, and storms on many hiking trips.
I would think there might be a number of things that would bring out clean water, at least from looking in the distance. I believe some have a tiny shrimp-like bug in the water.
Anyway, there is a close proximity to the source (snow and rain). Often filtered through rocks (like a sand-filter). No agricultural runoff, although some of the inlets may go through meadows, but even most of those would be slow flowing streams, usually with lots of plants and pools to filter out impurities, without artificial Nitrogen sources.
Perhaps the cold mountain weather also slows down the growth of algae.
If I remember right, the Selway Bitterroot Wilderness Area crosses the Idaho/Montana boarder. Most of the Idaho lakes are "Natural". Many of the Montana lakes have small dams artificially raising the water level. I think it made the banks a bit brushy.
And that first photo is a pretty picture... But many of those mountain lakes are awfully chilly, even mid summer!!!! I've gone swimming in them... about 30 seconds is all I can stand!!!