I have wanted a backyard pond for a very long time. Some people think of a pond as a 1/4 to a 1/2 of an acre or larger. I plan on putting in a small pond, something around 15 ft wide x 30 ft long x 1-6 ft deep. A small pond that size will hold an amazing amount of water, in this case over 10,000 gallons.
There are many reasons other than aesthetics to put in a water feature on your property. You will increase the bio-diversity of your property instantly. Frogs will start showing up, birds will stop by as well as insects, spiders, crustaceans, amphibians, reptiles, fish and mammals. By adding a water feature to your landscape you will create an environment that really can't be measured by a metric I know of. Water is truly the catalyst of life. You will have a healthier more diverse property if you create aquatic habitats.
A pond allows you to fish for your dinner as well as grow many water loving plants. I plan on growing many things in my pond. Did you know you can grow water chestnuts in the midwest? You can grow duck-weed to feed your livestock and to support the fish and other life in your pond. I will put in cattails which are almost completely edible, and what you can't eat can be composted. So even if the only thing you can do is put in one of the plastic ponds from your local big box store, I hope you will put some sort of water feature on your property.
One of the first things you should do is locate an area that will best support a pond. Things to consider are drainage areas, soil composition, low lands and proximity to buildings and septic systems. You don't want to locate your pond close to a building or a septic drain field / tank. My pond will be about 20 ft from my garage and 40 ft from the closest drain field. I am sure there are regulations on these distances from your local government overlords so check your local code.
The area I am locating my pond happens to be the lowest area in my backyard. When it rains a lot this area holds surface water for a couple days. Once you locate the best area on your property for a pond, you will want to dig a test hole. This will let you know what type of subsoil you are dealing with. About one foot down my sub soil turns into a nice clay layer. Clay is good, if you find your sub soil is gravely, you will need to seal your pond with a liner or bentonite clay. I have seen some people use old carpet, it works, but I know what goes into the manufacture of synthetic carpets, so I would pass on that.
The other purpose of a test hole is to see if your ground will hold water. I dug this hole as deep as I could with a post hole digger. I ended up getting it around 4 ft deep. Next you want to fill the hole up with water. Now you just wait to see how long it takes for the water level to drop. If you walk away to grab a beer and when you come back the hole is empty, then you may have trouble putting in a pond the easy way.
I checked the hole the next day and it had gone down 2 feet. The ground was dry when I did this experiment, so I filled the hole back up and it dropped about two feet in 24 hours. I assume this is because the top 2 feet of the hole allowed more water to seep into the ground than the tighter packed clay on the bottom. After about a week there was only a foot of water in the hole. I am going to ask my neighbor if he sealed his pond, but I don't think he did.
I am going to plumb the downspouts on the back of my garage into a water catchment system eventually. I will take the overflow from that, and using french drains, have the system drain into the pond. More on that in the future.
One last thing, put something like a board over the test hole you dig in case someone is walking around your backyard and doesn't know it is there. It would be quite a surprise to step in this hole and have your foot go down a couple feet.