For an early-summer backpacking trip, it’s hard to beat this 19-mile trek along the northern edge of Yellowstone [1]. Start at the Hellroaring Trailhead, 3.5 miles west of Tower [2], and follow the Yellowstone River Trail in a steep descent to the river, 600 feet below.
A suspension bridge crosses the river, and from here on you remain on the north side all the way to Gardiner. One ford at Hellroaring Creek can be dangerous before August, but it can be avoided if you hike a mile or so upstream to a stock bridge crossing.
After this point, the hike alternates between high ridges overlooking Black Canyon of the Yellowstone and quieter stretches where the trail drops down along the edge of the water. Dramatic Knowles Falls is a highlight.
The trail is generally in good condition, but you may find it hot and dry in midsummer. Check with the Park Service for the latest on access to the west end of the Yellowstone River Trail, since it passes through private land near Gardiner and that portion may not be maintained or accessible.
Links:
[1] http://www.moon.com/destinations/yellowstone-grand-teton/yellowstone-national-park
[2] http://www.moon.com/destinations/yellowstone-grand-teton/yellowstone-national-park/sights/tower-fallroosevelt-lodge