The central horse shoe valley in the hills used to be drained south by the Mahim river in the past. This river has been dammed in its upper reaches, so mruch so this valley today accommodates three small fresh water lakes, the Tulsi , the Vihar and the Powai , one below the other, that supply the city with 3 per cent of its domestic and other needs of water supply. Below Powai , the river today is mostly a storm drain and a gutter of sewerage, blocked off by the construction of the Santacruz airpoit at its Kurla end. The lower reaches is a shallow, fastly silting up drain of industrial wastes emptying into the Mahim Bay. The Kanheri hill complex has a radial drainage system, with numerous rain torrents washing down its slopes in all directions. The largest of them is the Dahisar river that rises on the southern flanks of the Kanheri hills, and drains west to join the Marve creek; this river, however, has been blocked to form the Dahisar project, to augment the water supply to the city to a small extent. Juhu , Varsova , and Marve , are sand bars pushed inland by the sea, and resting on knolls, Dharavi and Rai Murdhe . Today they form together a single land mass, off the Ulhas mouth. Surrounding the central hilly region, are extensive lowlying plains, that are reclaimed tidal marshes and flats just above tidal levels. They are widest to the west and south of the central hilly region, and narrowest to the north. In the east, along the Thane creek, sizable stretches of tidal swaps and salt pan areas, especially around Bhandup , still survive Creeks , Marshes , Tidal Swaps , Lakes And Hills