A serious koi pond should be at least 3 feet deep and no areas should be shallower than 2 feet. Any areas shallower than 2 feet is an open invitation to dinner for both blue herons and raccoons.
If you don’t think you have predators in your neighborhood just build a shallow pond and watch what happens! Even a single safety step to help you get out of the pond, that is less than 2 feet deep, can cost you most of your fish. Also, all sides should go almost straight down, not tapered. Aquascape type ponds where the sides of the pond gently taper in toward the bottom will not work for koi.
Even if you build a deep area with a cave in it for them to hide in, you are still not safe. Blue herons will wade out into the water, stand perfectly still for hours if necessary, and simply wait for the koi to come out and then strike. Do not place shelves for plants in the pond. We suggest if you want to put plants in your pond that you hang potted plants over the side with wires to support them. That way if you start having problems you can simply remove them. If you decide to ignore this advice you will probably (after loosing several of your favorite fish) end up covering your pond with a net or as with one case we saw, half inch rebar used to keep out raccoons.
Most people would agree that a beautiful pond is somewhat ruined when it has to be covered with something to protect the fish. A deep pond with steep sides is the best defense against predators. Also, koi get their exercise by swimming up and down in a pond rather than horizontally. The constant change in water pressure helps koi develop a proper contour or shape. Therefore, if you plan on growing jumbo koi you will need to build the pond at least 6 feet deep.