Portland’s Marine Drive has a bike path along several miles, but there is also a trail that can be walked that lies between the Marine Drive bike path and the river’s edge.
There is space for several cars to park at the west end of the Marine Drive bike path by 33rd Ave. An underpass under 33rd Ave makes it easy to park here and either walk or bike to the east.
Broughton Park has a large parking lot, with a fee for parking. From here, you have options for walking to the east. Follow the paved bike path, the dirt trail, or the sandy beach (depending on water level).
The dirt trail can be followed about a mile and a quarter to the east. The paved bike path goes about 4 miles to where it then connects with bike paths along I-205 going south and north crossing the Columbia River.
An interesting bike ride is to start at 33rd, follow the bike path east, then cross the Columbia River and turn around when you reach the neighborhood streets in Vancouver. This makes for a nearly 16 mile round trip on a separated paved bike path that only requires 2 crossings of Marine Drive in each direction, and no other street crossings.
Starting on the Washington side of the river, there is space for a few cars to park at the end of SE 23rd St. From there, follow the I-205 Multi-Use Path across the Columbia River. You can choose to take the path along Marine Drive, or south along I-205.