GNP is packraft legal. I spoke with a BC ranger a few days ago, and she was puzzled that it wouldn't be open to boating. She couldn't give Yellowstone an accounting for.
I did a neat two day loop that bears repeating. Road biked from Camas to Apgar, snagged a permit, rode the shuttle to the Loop, hiked over Flattop Mountain, floated the lower Waterton River, hiked over Browns Pass and down to Bowman Lake, avoided 6 miles of road walking by hitching a ride, then floated the North Fork of the Flathead back to the truck. I call it the North Fork 100 (as it's about 100 miles all told, and is a good shorthand). Video summary: http://vimeo.com/13442041
July's a great time to be out in the park. Some snow, but not too much (my ice axe was dead weight). Plenty of water in the rivers, but not at flood stage, and very clear water (still darn cold). And most importantly, the flowers run riot. Even though we had less snow than last year, it has melted off much slower due to a wet and cold June. Last year this time the meadow north of Fifty Mtn was the most flagrant display of wildflowers I've ever seen, this year it was a 1/2 mile snowfield. So, in other years runoff might be sooner and thus July water lower. Perhaps.
Waterton River beta:
Above Pass Creek the river looked (from the trail) and sounded pretty hairball. Some gnar looking chutes with tons of wood. About 1/3 mile past the trail to Stoney Indian you come right down to the river, and it looks very inviting. The stretch down to Kootenai Lakes is exactly that, very meandering, no riffles, and plenty of wood to get around (but only two river wide jams). Lots of waterfowl and beavers, and phenomenal scenery. Kootenai Lakes was a nice flatwater paddle with more of the above. Not far below the lakes the river got rowdy, with a 4-6' tall ledge diagonaling almost all the way across the river. It would go on the far right, but absent a PFD, helmet, partners, and road nearby I portaged river right. Below that the river remains fast, with a good number of mild boulder gardens that aren't serious, but do require paying attention and maneuvering. I took out when the river starts to curve east, and it seemed like things got steeper and more serious below that. A competent crew of rafters would likely have a ball running the whole thing, and even as I ran it hauling the boat back there was totally worth it. Really a tremendous setting.
North Fork beta:
Not much needs to be said that can't be found elsewhere. This was a great 16 mile, 3 hour ride on the raft-a-lounger, with only a few occasions when I had to put my feet back in and pay attention to picking the right channel and not getting sucked into a sweeper on the bank. Even though there are roads running on both sides, and thus plenty of fisher-people and floaters, the North Fork has a wilderness feel and more importantly, is gorgeous water in a gorgeous setting.
Bowman Creek above Bowman looked like it just had enough water to go, unfortunately it looked to have a channel wide log every 50 meters the whole way. I didn't bother. Bowman Creek below the lake would be worth a look, but the gradient and twistiness on the map inclined me to take the easy way out.
At least this time of year, the creeks in GNP don't seem to need too big of a basin to get enough water to float. Plenty of other places seem worth investigation, especially the lower Belly River and Nyack Creek.