The Tama River begins in the steep, wooded valleys at the foot of Mt. Ontake and Mt. Takanosu, to the west of Edo. The river then runs in a nearly straight line across the hilly plateau that forms most of Musashi province, flowing about 75 km (45 miles) before it reaches the shores of Edo Bay.

The Tama River is very important to the city of Edo. Not only does it mark the southwestern boundary of Musashi, it also is the source of much of Edo's water supply. A huge canal carries water for just over 40 km (25 miles) , from the headwaters of the Tama River to a resevoir in the north of the city. This resevoir supplies many of the wells and fountains in the downtown area, where people get the water they use for drinking, washing and so on. The river also supplies water to the many rice fields and farming villages along its banks.