The Clarkston Greenbelt Trail is part of the Clearwater and Snake River National Recreation Trail and is administered by the Army Corps of Engineers which constructed the dam. The northern end trailhead is in Granite Lake Park. Without the dam, you would be along the Snake River, just downstream from its confluence with the Clearwater River, and across the river from one of Lewis and Clark's campsites back in 1805.
The towns of Lewiston and Clarkston, sister cities, are named in tribute to the explorers, and each town has a trail on the bank of the rivers courtesy of the Corps of Engineers. Bridges near the northern and southern ends of the trail connect the Greenbelt Trail with the Lewiston Levee Trail and allows you to make a loop ride if you so desire. The Greenbelt Trail runs for 7 miles and the Lewiston Levee Trail runs for 11 miles.
The Greenbelt Trail's northern end is actually in the greenbelt of parks along the river. Be aware the trail routes you through several parking lots and down driveways several times, then continues beyond as asphalt trail. Enjoy the shade offered by the mature trees in this section...
...because the shade ends after about 4 miles after you go under the Southway Bridge, and you then find yourself out in the full sun.
The trail ends at Chief Looking Glass Park in Asotin, WA. As you near Asotin, the trail turns to the right and continues alongside the road to bypass the water treatment plant. It then bears left and drops back down to pass along the high school football field and track and enters the park.
All along the trail are informational signs, giving Lewis & Clark information, Nez Perce info, and info on the fish and floods of the river. An interesting sign at Granite Lake Park relates how Lewis and Clark almost slept here -- instead they camped across the river to avail themselves of the high hill there to get star sightings (but the clouds prevented them for doing so.)
Trailhead parking is available every few miles if you drive along the river. Specifically:
Granite Lake Park at 850 Port Way
Greenbelt Ramp at Fair St. and Riverview Blvd.
Swallows Park at Hwy. 129 and Hillyard Dr.
Hells Canyon Rec Area office at 2535 Riverside
Chief Looking Glass Park at Hwy. 129 and Clemans Rd (Asotin, WA)