Lake Mono, situated close to the CA 395, is about 14mi/23km southeast of the eastern entrance to Yosemite National Park and about the same distance south of Bodie. This salt-water lake, 6,240ft/1,900m above sea level, is 13mi/21km wide and 8mi/13km long. Although several rivers flow into Lake Mono, it has no outlets, and is one of the oldest lakes in the world, having been formed perhaps 700,000 years ago. In the water, which has a higher salt content than most lakes, only a few life forms can exist, mainly single-cell algae; these provide food for the salt-water flies and brine shrimp, which in turn are eaten by 70 kinds of migratory birds which reside on Lake Mono in spring and summer. These are mainly phalaropes (wading birds), grebes and Californian gulls. It is estimated that each year a million of these three species migrate through Lake Mono at various times. About 90% of all Californian born gulls are hatched on Lake Mono - especially on its volcanic island of Paoha and Negit Island.