The area from Orlando to the tip of the Florida peninsula was at one point a single drainage unit. When rainfall exceeded the capacity of Lake Okeechobee and the Kissimmee River floodplain, it spilled over and flowed in a southwestern direction to empty into Florida Bay. Prior to urban and agricultural development in Florida, the Everglades began at the southern edge of Lake Okeechobee and flowed for approximately 100 miles (160 km), emptying into the Gulf of Mexico. The limestone shelf is wide and slightly angled instead of having a narrow, deep channel characteristic of most rivers. The vertical gradient from Lake Okeechobee to Florida Bay is about 2 inches (5.1 cm) per mile, creating an almost 60-mile (97 km) wide expanse of river that travels about half a mile (0.8 km) a day.[24] This slow movement of a broad, shallow river is known as sheetflow, and gives the Everglades its nickname, River of Grass. Water leaving Lake Okeechobee may require months or years to reach its final destination, Florida Bay. The sheetflow travels so slowly that water is typically stored from one wet season to the next in the porous limestone substrate. The ebb and flow of water has shaped the land and every ecosystem in South Florida throughout the Everglades' estimated 5,000 years of existence. The motion of water defines plant communities and how animals adapt to their habitats and food sources.[25]