Lake Okutama(奥多摩湖, okutamako) is the biggest artificial lake in Tokyo and the largest one in Japan as Reservoir. Lying above the Ogochi Dam. The real name of Lake Okutama is the Ogochi Reservoir. Ogochi Dam was built in 1957 and it was the biggest dam in the world at that time. There are many tourist attractions around the lake. As the Oasis of Tokyo down town, many people visit lake Okutama to feel and enjoy nature.
Touris Info.
You can enjoy fishing and hiking around Lake Okutama. Many hotels, gift shop and restaurants are placed.
Access
0. at Tachikawa Station
1. use JR Ome Line to Okutama (1hr. 20 min.)
2. use Bus to Ogochi dam (15 min.)
2 hr.
New Wave Samurai Movie. 
映画好き集まれ！ 
One of the best nature spot around Tokyo. The Taba (Tama) River feeds Lake Okutama at its western end. From the southwest, the Kosuge River also flows into the lake. The Tama River drains the lake at the eastern end.
The surroundings are famous for cherry blossoms in the spring.
Ogochi Damu
Started at 1938
Finished at 1957
Surface area : 4.25 km²
Max. depth : 142 m
Surface elevation : 526.5 m