This sight is probably unusual:

• This viewpoint requires a little bit of effort to get to so there are not many people who see this scene in a year.
• The stream must be fairly full. Recent snow in the area could account for this. In a few days, the stream could be just a trickle.
• The stream does not flow very fast here because the canyon bottom is flat, hence the water pools and there are no ripples to spoil the reflection.
• There was no wind blowing (fairly unusual for Spring) so no wind ripples to spoil the reflection.
• Finally, the clear blue skies and late afternoon sun on the cliffs but deep shadow in the canyon bottom made the effect noticeable. We wouldn't have noticed it 30 minutes earlier, and 15 minutes later it had faded.