Hartley Nature Center has a large wetland with a beaver pond.
The Lake Superior and Mississippi Railroad passes many marshes along the St. Louis River.
The Minnesota DNR 
is working to restore wetlands on Grassy Point in the Duluth Harbor...
http://www.d.umn.edu/~pcollins/grassy.html
http://www.dnr.state.mn.us/rprp/grassypoint/index.html
..and also
Miller Creek, which flows from shrub wetlands in Duluth and Hermantown.
http://www.dnr.state.mn.us/rprp/millercreek/index.html
Southworth Marsh is a forested wetland on Park Point.
St. Louis River Interlake Duluth Tar Superfund site:
http://www.slridt.com/
W.J. McCabe Chapter, Izaak Walton League of America:
http://pws.chartermi.net/~duluthikes/wetlands2.htm
St. Louis River Citizens Action Committee:
http://www.stlouisriver.org/