Portage Creek is a beautiful and relatively unique body of water just half an hour south of Anchorage, AK. What makes it unique is that it is fed from the overflow of the 780 foot deep Portage Lake, nested deep in the Chugach Mountains.
Portage Glacier clings to the side of the mountain over this lake, frequently calving large icebergs into the lake. What this means is that frequently you can float the Portage Creek with fairly large glacial icebergs on both sides of you as the lake spills into Portage Creek.
For a "lower 48er" like myself, the prospect of floating a freshwater creek with glacial icebergs on both sides makes this trip worth it every time. Generally the creek is a smooth ride, and though there are a few bumps there is nothing that I would characterize as a "rapid".
As you continue to float northwest, the channel splits into two parts, and doesn't join again for about two miles. Either channel will provide you with spectacular views of the hanging glaciers of the Portage Valley. Moose, Bald Eagles, and good old Ursus Horribilus (grizzly bear) frequent the banks of the river so keep you camera handy.
Overall you can float for about six miles until the river lets out on the eastern shore of the Turnagain Arm. The scenery and wildlife always make this trip worth it.
Two Warnings... though boating is allowed on Portage Creek, there is a threatening $5000.00 fine for kayaking on Portage Lake, which is very tempting to do with all the icebergs. Put in at the bridge or later to avoid this. Also, at the take out beware the dangers of the Turnagain Arm, with its massive bore tide influx and quicksand like mud. Take out right at the bridge rather than paddling out into the Turnagain Arm.
The Begich Boggs visitor center has bathrooms and parking. For basic amenities you will have to stop at girdwood along the way or proceed all the way along Portage Glacier Road to Whittier.
From Anchorage, proceed south on the Seward Highway about forty miles until coming to the intersection with Portage Glacier Road, which heads to Whittier. Turn left (west) on portage glacier road and proceed about six miles until coming to the begich boggs visitor center. Parking is free here.
I put in under the bridge next to the Begich Boggs visitor center. There are several other possible put ins along the Portage Glacier Road.
A map of the portage valley or at least a google earth shot so you understand the different channels of the creek.