The original Everglades ecosystem covers more than half the distance from Florida’s northern border to its outlets in Florida Bay and the Gulf of Mexico. It stretches from near Orlando in the north to Biscayne and Florida Bays and the Gulf of Mexico in the south, to Indian River Lagoon on the east coast and San Carlos Bay on the west. These habitats include the sloughs and lakes of the upper Kissimmee River and its floodplain, Lake Okeechobee, the sawgrass plain, ridge and slough wetlands and marl prairies south of the lake, and ultimately the bays and estuaries of the Florida peninsula as well as the islands and coral reefs of the Florida Keys. These diverse landscapes are connected by the flow of water over more than 220 miles from north to south and across 18,000 square miles, representing most of southern Florida. The interior watershed is about the size of Connecticut and New Jersey combined; when you include the affected coastal waters, you can add Rhode Island and Delaware. 
Another crucial aspect of the greater ecosystem is its interconnectedness over vast areas. Historically, the Kissimmee-Okeechobee-Everglades watershed—the core Everglades system—covered about 9,000 square miles in a single hydrologic unit. Rainwater in the northern half of the watershed followed the Kissimmee River and its tributaries and drained slowly through large areas of wetlands into Lake Okeechobee, a shallow water body of nearly 750 square miles. During the wet season when water levels were high, this northern drainage spilled over the south bank of Lake Okeechobee into the Everglades—once a vast, pristine wetland of about 4,500 square miles that included what is now the more than 1000 square mile Everglades Agricultural Area.