In 1979, the Florida Legislature designated the Green Swamp Area of Critical State Concern. The area consists of approximately 322,690 acres with portions lying in northern Polk and southern Lake Counties. The designation recognizes its valuable hydrologic functions and the need to specifically regulate encroaching development that imperils these functions. The Floridan Aquifer reaches its highest elevation within the Green Swamp, providing groundwater pressure to help maintain free-flowing springs, rivers, and abundant high quality drinking water. Additionally, the wetlands in the swamp coalesce to form the headwaters of four major rivers in central Florida: The Withlacoochee, Oklawaha, Peace, and Hillsborough Rivers. The Peace and Hillsborough Rivers are potable water sources for large population centers such as Tampa and Sarasota, and the Oklawaha, Withlacoochee and Hillsborough Rivers are designated Outstanding Florida Waters. The wetlands in the Green Swamp retain the seasonal flood for extended periods of time beyond the rainy season which reduces peak flows and flooding, increases aquifer recharge, and helps maintain seasonal river levels. The flatwoods and sandhill uplands that exist throughout the swamp provide moderate to high recharge to the aquifer. Moreover, within some areas of the western portion of the swamp, the limestone aquifer outcrops to the surface of the land, making it susceptible to pollution. For the reasons noted, the Green Swamp is considered an important hydrological area second only to that of the Everglades.