Making your pond more shallow: "I have koi in a pond that is 5 feet deep. I find this very hard to manage from the stand point of the amount of chemicals needed if I have a problem and hard to catch fish if needed. Can I fill the bottom with gravel to raise the pond floor level and to change the amount of water needed? What is the depth needed to maintain your fish? (answer) I am in Tennessee and we do have cold winters."
This is not an uncommon problem. People make pretty deep ponds. Without really top notch filtration, it's sometimes hard to get real clarity down that deep. Still others can't catch fish for handling and medication when they can retreat down to five or six feet. So I understand your situation. However, DO NOT USE ANY MEDIA TO REDUCE THE DEPTH.
Gravel, or sand, or actually any other aggregate or material on the bottom of the pond may slowly accumulate organic (carbon based) fuel within it. Eventually, without Koi to churn and sift lighter gravel, water flow may cease to percolate through the media and the sand, gravel etc. will "go anaerobic" meaning no oxygen will exist in the depths of the sand or gravel. This is UNCOMMON in sand or gravel under two to three inches deep, and even less common if plant roots, Koi and other copepods keep the media "lively" and turned over.
But if it does compact; anaerobic bacteria will begin to use sulfur for energy instead of carbon. Hydrogen sulfide will be produced and your fish will begin to suffer disease and death.
I saw a pond which was eight feet deep. They'd used four feet of sand to create a manageable body of water. In the sand, thirty months after its application, there were pockets of anaerobic gas formation. The fish showed typical Hydrogen Sulfide poisoning, in the form of dizziness, spiral swimming, gasping, and death. All the fish did eventually perish.
Avoid the use of ANY media on the pond bottom which can trap and hold debris, dirt or wastes for any length of time. Use of any material on the pond bottom will eventually cause trouble unless it's either turned over by the Koi, or turned over by a cleanout in the Spring. If you have a gravel bottom pond and don't maintain it, you can be responsible for your own results.
In your case where you have an enviable pond depth, and lot of water, a seine net will be of great value if you want to catch the fish. This is run in the pond at one end and dragged across and under the fish. Pull the fish up the side and into your waiting nets. Be gentle. Seines are available lots of places. Memphis Net and Twine might be a good starting point.