I am going to read back on your project Bob. Sounds fun!
As with most things, you can have too much of a good thing. Including pallets. First, pallets are usually treated. Not usually enough to do any damage, but the potential is there. They will gradually degrade, but not for years. Second, pallets catch hooks. Snags. A lot of folks like PVC structures instead. An array of pallets in different shapes and sizes will be fun to put together and place. Try to mark them so they're easy to find when the pond fills up. Concrete and rocks are great! I don't know if you can have too much rocky substrate. But it's definitely labor intensive. I encourage you to check out the structure thread that Bill Cody posted earlier. Great ideas there.
Edit - With regard to the concrete and rocks. As the pond silts in, as they all do (Lusk says every pond wants to be dry land), a large majority of your rocky intentions will be covered in mud/silt. So on the bottom of the pond, unless it's a pile, kind of defeats the purpose. But on the shoreline, like the first 4-6 feet deep or so, is wonderful habitat for an array of species. Bouncing jigs off rocks is serious fun too. All depends on how much work you want to put into it.
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