Most deer breeders and hunters alike typically think that all you need to make a watering hole for their animals is an excavator and an area to dig a hole in the ground. However, this is not the case when creating a healthy water source. The aquatic midge that causes EHD thrives in water sources that are shallow, have a degraded water quality, and have a soft and organic bank to breed in and around. The main rule of thumb when creating a pond, lake, or water source is that the more water you have the easier it is to manage. Why you ask? The simple answer is that when a water body increases in size and depth the heating and cooling potential dramatically decreases.
The easy way to think about temperature of water compared to water body size is of a swimming pool. As summer progresses along you may begin thinking that the weather is nice and warm so it should be time for a swim. However, as you jump into that pool in May you get shocked with a very cool sensation rushing over to you as the water has not heated up to the same temperature as the air around you. Now on the flip side you may go on that same day and play in a puddle which may be exhibiting equal if not higher temperatures than that of the air. Making sure to keep the comparison of water temperature to water body size and depth will help you realize why those shallow puddles you may have created could be a midge fly threshold.
A properly designed water hole/wildlife pond should have steep slopes, but keeping in mind that a deer should still be able to walk in and out of the pond rather easily to avoid stress on the animal. A 3 to 1 slope is typically a safe bet to not only give the deer an easy access in and out of the pond, but also be able to establish beneficial aquatic plants. In addition to the slope making sure the pond reaches a proper depth is very important. To help with management of the pond a depth of 4+ feet should be achieved. This would make the pond 24 feet in diameter. To increase the depth, you will either make a larger diameter pond or increase your end slope in areas of the pond. In summary the deeper and steeper you make the pond the easier it is to manage.
Now that you understand the design of the pond you must now look at how the pond will hold water, and how to reduce the amount of muck/soil that the midge can breed in. In many areas across the Midwest it can be hard to find a spring to maintain your water level. In those cases, a synthetic liner can be your savior. Not only will the synthetic liner help maintain water, but it will also eliminate the loose muck/sediment that the midge needs to breed and reproduce. Typically, a material that is 24 mil or greater is sufficient in holding water meanwhile being strong enough to prevent rips/tears from deer hooves. Furthermore, unlike the typical bentonite clay that people have used to hold water in their ponds/waterholes, the synthetic liner does not leak as easy, and leaks are much easier to locate and repair.
The Catch-22 to this idea is that having a full functioning ecosystem is still key in maintaining superior water quality. To achieve this goal a layer of sand is recommended to be placed on top of the liner. This will help become the planting substrate for your beneficial aquatic plants. The sand will also help to cushion the liner and create a substance in which wildlife can walk freely on with reduced or non-existent slipping. The last item the sand will help with is in prevention of UV rays hitting the liner which would breakdown the plastic of the liner.