This 8 mile paved path runs on a levee between the Columbia River and Marine Drive, at times a bit above the roadway or bit below. You see the river nearly the entire ride, with views of Government Island State Park as you near the I-205 underpass, and views of Vancouver, Washington across the river as seen below as you head west...
I believe the 8 mile length includes another segment of trail to the east which would require riding on the road to reach, but for an interesting change of pace and some additional mileage on a bike trail, when you reach the eastern end of the trail at the I-205 bridge, take the connector trail and you'll find yourself heading over the bridge to Washington, with four lanes of traffic on each side of you and low flying airplanes above you as they head to or from Portland Airport, which you pass on the Columbia River Trail. The bridge is over 2 miles in length, uphill as you head north, so you can get 4+ more miles as well as a bit of a workout.
Though a bit noisy and smelly from the vehicles, you get a cool breeze on the bridge (and even gusts which signs warn you about) and you also get amazing views of the magnificent Mount Hood in the distance as seen below (click to enlarge)...
Finding trailhead parking is tricky. There is a small gravel lot off NE Marine Drive and the I-205 bridge.
On the west end, there is a marina lot and also a large gravel lot across the street, both about 8/10ths of a mile east of NE 33rd Drive.
Access to Marine Dr. is limited. From NE Lombard St. you can get to 33rd and then to Marine Dr. and then turn right.
From Sandy Blvd. you can take 122nd or 138th to Marine and then turn left.