South Bass Trail is a 12.5 mile lightly trafficked out and back trail located near Grand Canyon, AZ that features a river and is only recommended for very experienced adventurers. The trail offers a number of activity options and is accessible from April until October.
This is a strenuous, 15.6-mile (round-trip), remote, non-corridor hike in Grand Canyon National Park. Camping here is at large, as there are no established sites below the rim. There is also no water at the trailhead--not until you get to the Colorado River more than 4,400 vertical feet below and 7.8 trail miles later.
This is, so far, my favorite hike in Grand Canyon. The tricky part is getting to (and from) the trailhead, which involves a long drive on unpaved Forest Service, Reservation, and National Park roads (with a $25 fee required at the gate to the 1.8 miles of road through the Rez), which can be impassable in winter and wet weather. The hike, though, is just amazing, with great camping at the trailhead (no water or bathrooms), on the Esplanade--a good place to cache water for the return hike--and along the Colorado River. The descent through the Redwall is spectacular, and Bass Canyon offers lifesaving shade during the intense heat of summer.
Be sure to obtain a backcountry permit before doing this hike. I'd recommend carrying at least two gallons of water per person for the descent and caching one (or more) of those--again, per person--on the Esplanade. I'd also take a filter to obtain water from the Colorado River for the hike out, and bring a bandanna or some other method of pre-straining the water to remove silt.
You can see my full trip report, along with photos and information about accessing the trail, at http://www.squidoo.com/grand-canyon-south-bass-trail