On State Route 167 in Auburn, exit at 15th Street NW. A very short distance just east of 167, take the first road on your left, which is M Street. Turn left onto M Street and go about a half mile or so north. The marsh is on your right. SR 167 is on your left.
To see the location on Google maps, go to Google Maps and paste these coordinates into the search box:
Location of M Street Marsh in Auburn
15th Street NW Exit off SR 167