Regardless of the size, shape, depth or location of your pond, the first imperative of “winterizing” is a cleanout. Any organic debris (leaves, twigs, dead fish, etc.) left on the bottom through the winter will decompose, rot, and produce noxious gases, primarily hydrogen sulfide, which will poison your pond as the winter progresses. Inorganic debris, such as rocks or metal racks, won't decompose, but if they have sharp edges, they pose the risk of injury to your fish at a time when they are incapable of healing.
When to get started
Get into your pond in late summer or early fall, when the air and water are still warm if possible, and get all the gunk and junk out. A bare liner bottom gives you a definite advantage here, especially if it is equipped with a properly installed and engineered bottom drain. Rocks on the bottom do look more natural, but are impossible to clean thoroughly, and will retain a sizeable amount of sludge.
Once your pond bottom is as clear of debris as you can get it, find a way to protect your work. Remember, it's fall. The trees are just waiting for you to blink so they can drop another ton of used biomass directly into the pond you just spent all day cleaning. Leaf netting is widely available in most pond supply businesses and it needs to be supported well above the water surface so your fish do not get tangled in it.
It also would help to know how deep the pond is, but if it is 3 feet deep or deeper, it will probably not freeze through. You will need to keep a patch of water open (2-3 square feet should do it). We use an air stone attached to a powerful air pump. A dome made out of Styrofoam wreath forms and covered with clear plastic sheeting (at least 4 mil is best) with the air stone fed through the top and located about a foot below the water surface works well. The constant motion of the water will keep the pond surface from freezing over. The clear area will also give you access to the pond for small water changes every two to three days through the winter, feeding your submersible pump through the hole in the ice and pumping out a modest portion of the ickiest bottom water with each exchange and replacing it with fresh.(Don't forget to dechlorinate!)
Another solution to this issue would be to mate the air stone with a trough heater (available from your local Tractor Supply). These are electric heaters used by farmers to keep watering areas free of ice for livestock during the winter, and they will work in your pond as well. They need to be inspected very carefully for corrosion each season since they draw a fairly high current, and any water leak into the wiring will cause a short circuit that can injure your fish and also blow out your house wiring if not properly connected to a ground-fault interrupter protected outlet. We have heaters specifically designed for hobbyist ponds.
Your pump and piping should be stored dry to prevent expansion damage as the water freezes. Your filters stop being effective at water temps below 40 degrees. At that point, the filter should be drained and rinsed with pond water and allowed to go dormant. Water is at its densest at 38 degrees. A deep pond will stratify with 38-degree water on the bottom and your koi will congregate there, that temp being about the coldest they can comfortably tolerate.
You can leave the marginal plants where they are. They're hardy in the Louisville KY area. The hardy water lilies should be pruned back and sunk in the deepest part of the pond. They'll be fine as long as the corm does not freeze.
Wow, that's a lot of work you say! Well never fear, Two Fish Louisville is available to assist in any or all steps of this process. Every pond is different some winterizing practices may not be necessary for your application. Give us a call and we will come out for a free quote!