I did what Shelby County described above. Also took a bunch of tires and wired them together 'poker chip' style in order to create an artificial hollow 'log.' Got that idea from seeing them in ponds at a nearby state fish and wildlife area.

The tire structures aren't that close together although the pond is only 1/2 acre. Not sure how effective they've been since I keep my pond 'wild' and there is a lot of organic structure.