The floor of the pond goes in first. Start at one end and place the concrete to the full thickness of the floor-tamping it in with special care at the corners-and back under the overhanging wall forms to the outer reaches of the excavation. A straightened-out garden hoe is an excellent tool for this job. The weight of the concrete will probably dislodge the reinforcing steel. Reach down through the wet concrete, lift the steel back into position, and tamp the concrete snugly around it as you go.

I want to stress the word place, for that is exactly how the wet concrete should be put into the form. If you scoop it up in your shovel and throw it in an arc into the form, you will completely undo your tedious job of mixing. The concrete will separate in mid-air. The gravel or crushed stone will land first, and the mixture of sand and cement will follow. The result will be a weak, and probably leaky, concrete shell.

Fill in the floor to the full length and width of the excavation. When it is 6 inches thick, it should just touch the bottom edges of the wall forms. If the pond outlet is set slightly below floor level, work the concrete into a gentle grade that slopes down to meet it. Tamp the finished pond floor to a depth of half an inch or so to submerge the larger pieces of aggregate and allow the concrete to present a smooth surface.

The natural finish of the pond floor is smooth enough for most pond builders. However, if you want an even smoother finish, work the floor over with a wooden float as soon as the concrete begins to set.

Few pond builders bother to finish any of the concrete except what will show above the water line after the pond is filled. Below the water line, in a healthy pond, the sides, whether rough or smooth-finished, will soon be hidden under a layer of algae.

When the pond floor is placed, fill the wall forms, tamping the concrete with special care at the point where the wall joins the floor. Ponds that leak usually do so at this point. A good, solid bonding of the concrete there will prevent it.

Work around the perimeter of the pond, adding only 6 or 8 inches of height to the walls each time around. As each layer is placed, jab the tamping hoe vigorously between the inside form and the concrete to work the aggregate back into the mass and give the walls a smooth face.

When the side forms are filled, finish off the top surfaces with a wooden float as soon as the concrete has begun to set. The top edging need not be finished, of course, if you plan to decorate it with brick, flagstone, or cast concrete coping.