I've floated this a few times before. It's a fun, short, relatively close to Anchorage trip (about 6 hours home-river-home). The put-in is below the first bridge crossing Granite Creek prior to the campground. The second bridge crossing is an alternative put-in for an even shorter trip. The takeout is the parking area/ramp above the Six Mile whitewater section. I usually drop the raft and gear, drive to the takeout, bike the 4.5 miles back and stash the bike in the woods to be picked up after the float. Unfortunately, the section above the bridge is more suitable for kayaks and packrafts.
From the first bridge until past the campground Granite Creek is somewhat narrow and fast. Once Six Mile joins and doubles the water the river is substantially wider. Wood can always be a factor on this section so be ready to avoid. On 5/29/16 there were no hazards that could not be avoided. Total float time is maybe 1 1/2 hours. There are many gravel bars for lunch or a nap in the sun.