Natural vs man-made lakes

I can't give you a percentage, but I can tell you that there are a handful of geological processes by which natural lakes are formed. The process of recent glacial retreat is the most common source of natural lakes in the U.S., and is responsible for many of the lakes in the Midwest and Northeast, for example (and the upper Midwest especially so). So, for example, you can contrast Pennsylvania--which contains a number of natural lakes produced by glaciation--with Maryland to the south, outside the range of glaciation during the last ice age and which contains no natural lakes at all. The younger Sierra Nevada and Rocky Mountain ranges out west also experience(d) alpine glacial processes that contribute lakes, but the much older and hence shorter Appalachian mountains out east do not have a history of recent alpine glaciation. Volcanic lakes are less common, but you can find them in the Northwest. And you can also find lakes produced by fault activity (the Salton Sea in California) or oxbow lakes near rivers, which is responsible for some of the few natural lakes found in otherwise not-lake-prone areas such as the Southeast (e.g., Reelfoot Lake in Tennessee, and Lake Chicot in Arkansas, both formed from the Mississippi River). I imagine that factors such as rainfall/watertables and soil permeability also enter into the equation in determining how long-lived and stable such lakes will be. Even if you might get the occasional oxbow formation in a state like Texas, the smaller size of most of its rivers and its lower rainfall would mean that such "lakes" would actually be short-lived ponds. You find oxbow lakes known as "resacas" along the Rio Grande valley, for example, http://www.rbwi.org/what-we-do/watersheds-action-programs/rescuing-resacas. And even in a state with a goodly number of natural lakes (such as Ohio, which counts 110 natural lakes larger than 5 acres) there are probably as many or more man-made lakes. I can't find a good listing of the total number of manmade lakes in Ohio, unfortunately, but you might find it interesting to look at thishttp://www.dnr.state.oh.us/portals/7/pubs/Open_File_Rpts/05_Natural_Lakes_in_Ohio_1991.pdf and compare it to your assumptions, only to discover that perhaps several of the lakes you assumed were natural are actually artificial.

I suppose one term to use in your searching might be "reservoir" instead of lake. Reservoirs are usually large, man-made lakes used as water supplies for local/regional areas. Also, any lake with a dam (both concrete/engineered and earthen) or spillway at one end is usually indicative of a man-made lake. Here in Indiana, though we have a good number of natural lakes, the largest ones are actually reservoirs. And, there are a surprisingly large number of private man-made lakes using earthen dams.

This is a question that I have been interested in for a long time. I feel fairly sure that where I live in the southern Appalachian mountains there are no natural lakes, and I attribute this to the fact that these mountain are so ancient. Any natural lake will have long ago broken it's weirs. There must be geologists out there who know the answer.

there are a handful of geological processes by which natural lakes are formed. There are also other biological processes besides man -- the mighty beaver has done a tremendous amount of terraforming.

http://www.mgs.md.gov/esic/fs/fs15.html

Presumably, nearly every man-made lake or pond has a dam. Other than size, there is really no distinction between a lake and a pond. http://en.wikipedia.org/wiki/List_of_dams_and_reservoirs_in_the_United_States So, there are about 75,000 man-made lakes and ponds in the United States. (This would include natural lake and ponds that are man-enlarged by means of a dam, but let's consider those to be man-made bodies of water as well.) So now to answer the percentage question, we need the total number of lakes and ponds (artificial and natural) in the United States. Wikipedia has a "list of lakes" page for each state, all of them linked http://en.wikipedia.org/wiki/List_of_lakes_of_the_United_States. Some of these pages list a total number of named ponds, lakes and reservoirs (2340 in http://en.wikipedia.org/wiki/List_of_lakes_in_Arkansas, 3000+ in http://en.wikipedia.org/wiki/List_of_lakes_in_California; Minnesota, land of 10,00 lakes, has http://en.wikipedia.org/wiki/List_of_lakes_in_Minnesota, etc.) while others do not. I'm not finding any page that contains an estimated total count for the US number of lakes and ponds. But it seems reasonable to guess that the percentage which is artificial is surprisingly high. I would not be surprised if the total number of lakes and ponds is under 225,000, meaning that more than 33 percent are man-made.

There are a few types of artificial lakes that don't require dams, like old quarries. ( Coal ash ponds and wastewater treatment facilities are probably right on the edge of the definition of artificial lake.) And there are a number of dams that still exist in place but no longer impound water. So the numbers of artificial lakes are probably somewhat different than the number of dams, but I agree that number is a good place to start.