Here's a variation of cyclotouriste's route:
Route thru beautiful Gresham
Two summers ago I rode that route twice each weekend as part of a 56 mile training ride for the STP. Like cyclotouriste's this one is all on decent bike paths or quiet back streets. Near the last marker on the west side of Roberts St. you'll find an entrance to Main City Park. Head down the steep hill and on into the park and soon enough you'll encounter the Springwater, though you might have to hunt around a bit at first.
FWIW this connection between Marine Dr. and the Springwater was gleaned from the excellent "Bike There!" map available at many bike shops as well as Metro's website.