The geography of Toronto, Ontario, covers an area of and is bounded by Lake Ontario to the south; Etobicoke Creek, Eglinton Avenue, and Highway 427 to the west; Steeles Avenue to the north; and the Rouge River and the Scarborough–Pickering Townline to the east. In addition to Etobicoke Creek and the Rouge River, the city is trisected by two minor rivers and their tributaries, the Humber River in the west end and the Don River east of downtown. Both flow southward to Lake Ontario at Humber Bay and Toronto Harbour respectively, which are part of the longer Waterfront, as well as Etobicoke Creek and the Rouge River. The concentration and protection of Toronto's ravines allows for large tracts of densely forested valleys with recreational trails within the city. Approximately 26 to 28 percent of Toronto is covered with over ten million trees, a fairly high percentage within a large city in North America and there are ambitious proposals to double the coverage. Some parts of Toronto, such as High Park and the lower Humber River, are located in the northernmost parts of the Carolinian forest zone found in North America. The shoreline of the former Lake Iroquois is a major east−west geological feature, which was formed at the end of the Last Glacial Period. In the west end, Davenport Road follows the ancient shoreline with the steps to Casa Loma rising above and downtown skyscrapers clearly visible to the southeast. It merges with the current Lake Ontario shoreline at the Scarborough Bluffs promontory. The Toronto Islands are the only group of natural islands located on the western shores of Lake Ontario and were formed from the erosion of the Scarborough Bluffs. The Toronto Islands were originally a sand spit until a major storm separated them from the mainland during the 19th century. Toronto's immediate neighbours are Mississauga and Brampton within the Regional Municipality of Peel, Vaughan and Markham within the Regional Municipality of York, and Pickering within the Regional Municipality of Durham. The Greater Toronto Area (GTA) includes the regional municipalities of Halton, Peel, York and Durham. The GTA is part of a larger, natural ecosystem known as the Greater Toronto Bioregion. This ecosystem is bounded by Lake Ontario, the Niagara Escarpment, and the Oak Ridges Moraine, and includes many watersheds that drain into Lake Ontario. In March 2005, the Government of Ontario unveiled the boundaries of a greenbelt around