An illustration of Biscayne Bay, is a lagoon that is approximately 35 miles (56 km) long and up to 8 miles (13 km) wide located on the Atlantic coast of south Florida. It is usually divided for purposes of discussion and analysis into three parts, North Bay, Central Bay and South Bay. North Bay separates Miami Beach on its barrier island from Miami on the mainland. It has been severely affected over the last century by raw sewage releases, urban runoff, shoreline bulkheading, dredging, the creation of artificial islands and the loss of natural fresh water flow into the bay. North Bay accounts for only 10% of the water area of the bay. Central Bay is the largest part of the bay. It is separated from the Atlantic Ocean by the Safety Valve, a series of shallow flats separated by tidal flow channels, stretching from the south end of Key Biscayne to the Ragged Keys at the north end of the Florida Keys. It has been adversely affected primarily by bulkheading, urban runoff discharged by canals, and the loss of natural fresh water flow. South Bay is nearly as large as Central Bay, and is the least affected by human activities, although it also suffers from the loss of natural fresh water flow. South Bay is separated from the Atlantic Ocean by the northernmost of the Florida Keys, and is connected to Florida Bay through channels and "sounds" lying between the mainland and the keys.