Ogochi-damu-ko, or Okutama-ko, was formed by the construction of Ogochi Dam in the upstream of Tama River to serve as a reservoir of drinking water for Tokyo metropolitan area. It extends for 4 km in east-west direction, is 1.5 km wide (north-south), and has a water surface of 4.25 km². The lake is said to be the world's largest among the reservoirs used solely for city water supply. The construction work needed 19 years from November 1938 to November 1957.

Twenty percent of the lake's catchment area belongs to Tokyo Metropolis and the rest to Yamanashi Prefecture. The catchment is mostly mountainous, ranging in altitude from 2,018 m on the top of Mt. Kumotori to about 550 m at the lake surface. Flat lands are limited only to the immediate vicinity of the lake. Mountain slopes are mostly covered by mixed beech-fir forests and evergreen conifer (Cryptomeria) plantations (Q).