In Rocky Mountain Park and some other places in CO the streams are at high altitudes.
If you are not familiar with altitude sickness, do some reading online about it before you go.
When planning your trip, it's best to fish and hike etc. in lower altitude places in the first few days.
Then if you want to explore higher altitude places, do that in the later days of your trip.
Because your body adjusts to the higher altitude by producing more red blood cells. And even though you won't get fully acclimated in 5 days, even in 2 or 3 days, you will feel an improvement in that time.
And just don't push the altitude thing too much. Listen to your body, as the saying goes. If you're getting bad headaches etc., return to lower altitudes.
In Rocky Mtn Park, there are some streams in the lower altitudes (low for RMNP) and we had OK fishing there.
But we also hiked into higher elevation streams. We were fishing at over 9000 feet and the furthest up we got the stream was over 10,000 feet.
And by the time we got back to the car we were really exhausted and not feeling well at all.
That was pushing it too much. Not really safe. We then went to southern CO and fished some streams down there, which were at lower elevations (for CO).
Near the end of the trip we went back to RMNP, and hiked up to the same streams at over 9000 feet, and briefly over 10,000 feet. This time, because we had been in CO for about a week and had got acclimated to some extent, fishing at that altitude was not so bad. I felt much better.