I'm building my first pond now (see attached). I am using 4x6x10 pt lumber. My yard is slanted so I had to dig down to get a level surface. These boards are very heavy. I am running rebar through them and going to pour concrete into the holes. The lumbar is $20 each at Home Depot. I need about 28 so the cost is < $600. Normally I would not get lumber at Home Depot, but after bringing the first 8 home with my car (and having one crack the wind shield) I went to HD so I