Andrew,
The trail surface is paved for a few miles at each end and packed gravel in the middle. The gravel sections are not suitable for narrow tires. There is also a 1/4 mile section of gravel road between the end of the pavement on the Banks side and the beginning of the gravel trail.
Here's a link to the OR State Parks page for the trail; there is a link there to the trail map which shows the paved and gravel sections.
http://www.oregonstateparks.org/park_145.php
Dave