Six percent of the earth's land surface (3.5 million square miles) is classified as wetland. To put that in perspective, that is an area just slightly smaller than the entire United States, including Alaska and Hawaii. More than half of that total lies within tropical or subtropical latitudes in rain forests, river deltas, and coastal swamps. The vast majority of the remainder is locked up in boreal (northern) peatlands. Canada alone has fully 15 percent of the world's wetland acreage.

Many factors influence the distribution and extent of wetlands:

1. Seasonal fluctuation in rainfall and snowmelt that contribute to yearly flooding

2. Low-lying topography that collects surface water

3. Drainage properties of soil that either allow water to quickly seep away or trap it in place

4. Glacial history that translates into an environment pocked with depressions

5. Mean temperatures during the growing season that affect evaporation and plant growth

The interaction of these parameters determines the potential for wetlands, as well as their specific nature. Any one of them can encourage or curtail the possibility of wetland formation.