When I lived in Raleigh, NC, the house had a babbling brook and adjacent marshy area. I didn't try everything mentioned above, but the only thing that I tried that made a difference in the mosquito population was hiring a pest control/mosquito spray service that came out every 2-3 weeks to fog the perimeter of the property for like $75 a visit. I don't remember what the active ingredient in the spray was, but it's consumer grade and can be purchased/applied by you with a pump sprayer. Read the directions or it won't be as effective (don't apply within X hours of rainfall, don't overdilute, etc).
I already tried that with minimal effect. Was really disappointed.