The other route previously mentioned through Ladd's is the best way to go in my opinion, but if you like to switch it up every so often this is an easy one.
Cross over the Hawthorne, head south on the esplanade....get on the springwater and get off at the oaks bottom wildlife refuge. This will put you on SE Milwaukie.
Take a right on Milwaukie. You can take Milwaukie straight down to Bybee or you can cut over a block to 17th (much safer). Make a left on Bybee. Bybee will cross over 99 and then the road pretty much turns into Woodstock. Once you go through downtown woodstock, you will merge right on to foster. It pretty much turns into Foster right there. After passing under 205 you will see where the Springwater crosses Foster. Turn left and ride the springwater the rest of the way.
This part of the springwater is pretty bumpy for a bit. But I don't mind it, it is better than other sections. Riding Foster cuts out the real rough sections and it is more faast paced, not so many intersections.