A perspective from a local Calgary resident:
I'm a 40+ year resident of Calgary and I just happen to have planned a 5 day trip to Banff and Jasper beginning June 27.
I travel to Banff several times every year. I've also been glued to the local new the last few days. Fortunately my house is not in a flooded area.
The Trans Canada re-opend this morning between Banff and Canmore with limited access.
This info is from the town of Canmore website.
So cars are getting through but you can expect some delays.
If for some reason they have to close it again you can still get to Banff from Calgary via Red Deer and Saskatchewan crossing
http://tinyurl.com/l2ga5km Granted it would take about 4 hours longer than usual.
As for the Calgary Stampede. I have confidence that the Stampede will run on time. Some of the larger buildings may be damaged... but most of the fun stuff is outside anyway. I expect that the Saddledome will be closed. The only events that occur in the Saddledome are big concerts and some marching band competitions. I usually give those a miss anyway.
So I would not cancel you plans!