The wind blows 365 in the west-if you can borrow a 6wt-do so
load up on joes hoppers but wait until you get there-to buy-that's not lite wire dry country.
I never fished rock creek but Madison is a good choice-end of august is right at the very beginning of The Time
If you can stall until mid sept.-do so
believe me two or three weeks later worth it
thats when browns start moving up river to spawn-
If you can't stall -go high
upper big hole
yellowstone,madison in the park
ruby river
you want to be up on the divide
missoula is downstream
guys from over there heading to Butte,Bozeman,Livingstone area
remember this is the west-can snow anytime from labor day on
Seen snow in August in Gallatin Canyon
The Indian summer is the best time to be alive in the big sky country
You will have to look elsewhere for glacier advice.
If you do nothing else
take my advice to go high pre labor day
a couple 100 feet elevation can make the difference between spectacular fishing and a Montana is over-rated whine but don't tell the whiners because they know better-lol