1. Just a mindful thing you need to know about concrete, where other in the construction industry may hammer me on this..don't shoot the scientist. Concrete, depending on what it's used for previous before it may have been recycled and reused as base material, can contain excess heavy metals, as well as unknown trapped pollutants from the activities (and/or soils) that you may not be aware of as you salvaged it for usages. That then get introduced into your pond. The concentration of the pollutants and metals may not be high to be alarmed about, but I don't know how long the concrete materials have been supporting the roadways, leeching and absorbing. Definitely your call. Concrete will bring the pH up. Those who make concrete pipes, as well as inert landfill owners, claim that cured concrete will not bring pH down or raise pH up. Beg to differ. Plenty of enforcement lawsuits I have involved over the years that prove otherwise.
2. Mesquite tree indeed will float like a log, if it's dead and has been bone dried for a long while. Over time, as the water seeps into the body, and the wood undergoes decomposition, it will remain sunken. You won't know how much weight to use to anchor it down unless you do a trial run on the buoyancy level of the dead mesquite. You may need a 1/2 top of weight to hold it down, or you may only need a pebble. Can't tell until there's water. If you require to anchor them is not to compromise your pond's bottom lining. Just weight on both sides, and lots and lots of ropes, or chains (for added weight).
3. As for the concrete culvert, same as #1,but in this case, what type of water have been going through the culverts? Low flow rate is worst than fast, since pollutants have more time to adhere to the porous surface/subsurface of the culverts over time.
4. For the vertical structures, you got something going there. Indeed you're crazy, but in a good way. I'll hand this to the pond's engineers and the experienced pond masters.
