These would later split from Manhattan to form a county. During the 20th century, the rural character of Westchester would transform into the suburban county known today. The Bronx River Parkway, completed in 1925, was the first modern, multi-lane limited-access roadway in North America. The development of Westchester's parks and parkway systems supported existing communities and encouraged the establishment of new ones, transforming the development pattern for Westchester. With the need for homes expanding after World War II, multistory apartment houses appeared in the urbanized areas of the county, while the market for single-family houses continued to expand. By 1950, the total county population was 625,816. Major interstate highways were constructed in Westchester during the 1950s and 1960s. The establishment of these roadways, along with the construction of the Tappan Zee Bridge, led to further growth in the county. Westchester County is located in the New York metropolitan area and Downstate New York, north of New York City and south of Upstate New York. It shares its southern boundary with New York City and its northern border with Putnam County. It is bordered on the west side by the Hudson River and on the east side by the Long Island Sound and Fairfield County, Connecticut. According to the U.S. Census Bureau, the county has a total area of , of which is land and (14%) is water. Where Pelham Manor meets Pelham Bay Park in the Bronx, the southern border of Westchester is just under from Columbus Circle in Manhattan. At over , Pelham Bay Park is the largest of New York City's parks, forming a substantial buffer between suburban Westchester and the urban Bronx, while Van Cortlandt Park to the west acts as a similar buffer. Westchester's Long Island Sound shore is generally rocky, interspersed with tidal mud flats, marshes and wetlands, as well as several natural and artificially-maintained sand beaches. Municipal and county owned parks provide access to beaches, nature preserves and passive and active waterfront recreational facilities. Several large harbors lie along the shore including Milton Harbor in Rye, Mamaroneck Harbor, Larchmont Harbor, and Echo Bay, and the upper and lower harbors in southern New Rochelle. A number of islands can be found off the Long Island Sound shore, most of which are located in New Rochelle. Davids' Island, the former location of the U.S. Army's Fort Slocum, is currently unoccupied but is slated for use as passive parkland; Glen Island, currently a Westchester Parks Department run beach and park, was one of the first amusement parks in the country serving as a summer resort at the turn of the twentieth century; Huckleberry Island is largely undeveloped, and has one of the largest rookeries in western Long Island Sound; Echo Island is owned and used by a private yacht club; Execution Rocks is the site of a 19th-century lighthouse listed on the National Register of Historic Places; Columbia, Pea, and Goose Islands are undeveloped; Clifford, Harrison and Tank Islands are part of the "Five Islands Park" and nature preserve; while Oak and Pine Islands are used as private residences.