http://en.wikipedia.org/wiki/Barlow_RoadYou can follow the overland route over the Cascades by leaving I84 at The Dalles, takinng US197 south to Tygh Valley, and county and FS (48) roads west to OR35 (and on to US26 to Portland). The Barlow Road parallel to FS48 can driven in a suitably small vehicle (e.g. Wrangler).
Something I learned from the history of the Barlow Road was the importance of water sources along this wagon routes. Where possible the route ran from one spring to the next, if they weren't following a river.
http://www.nps.gov/whmi/forteachers/upload/Section%201%20Guide%20Oregon%20Trail%20Highlights.pdf Tolls on the Barlow road were $5, a week's wages. But the cost of rafting the Columbia was 10x that.