Prairies, whether they're "wet" or "dry" based more on what plant species grow there than on the actual amount of water present. Wet prairies occur on mineral soils that are covered with water about 50-150 days per year. The water rarely gets any deeper than about 20 centimeters even in the wettest of seasons.
Wet prairies usually burn at least once during a five year period. Without fire, woody plants would push out the prairie plant species. This would in turn cause prairie mammals and birds to lose their habitat.