About 80% of the water sources held by the Tokyo Metropolitan Government lie in the Tone and Arakawa river systems, according to the Bureau of Waterworks' Management Plan 2026. The Bureau states that those systems were developed to cope with a drought of a scale seen about once in five years, a lower safety level than other major Japanese systems.
The plants tell the same story
Purification capacity confirms the dependence. As of 31 March 2024, plants drawing on the Tone and Arakawa systems accounted for 5,480,000 m³ a day of Tokyo's 6,844,500 m³ a day, or 80.1%. The Tama River system accounted for 1,164,500 m³ a day and the Sagami system for 200,000 m³ a day.
Capacity is not the problem
Demand has barely moved. The Bureau distributed 1,540,566 thousand m³ in FY2025, only 0.7% more than in FY2016. On the peak day of FY2023, distribution reached 4,476,500 m³, about 65% of installed purification capacity. Tokyo's exposure is the reliability of raw water in a dry year, not a shortage of treatment plants.
Help depends on national projects
The Tone basin projects named in the plan are national ones. Yamba Dam was completed in March 2020, and the Kasumigaura Water Conveyance Project is due in FY2030. The Bureau's own levers are mostly on the Tama side, including the Ogouchi Reservoir, more than 60 years old, and 25,444 ha of water conservation forest. Since 1 April 2026, a national tap-water standard of 50 ng/L for PFOS and PFOA combined has applied, and the Bureau stops intake from any Tama well that risks exceeding it.
How the drought standard sits beside the seismic program, who decides on water quality and basin projects, and how renewal is funded at current tariffs are set out in the full report. The full report, Water Systems Overview: Tokyo Metropolitan Government Bureau of Waterworks, is available for USD 499 at https://ourfuturewaterintelligence.com/reports/water-systems-overview-tokyo-metropolitan-government-bureau-of-waterworks. For a free overview, read the Tokyo Metropolitan Government Bureau of Waterworks Utility Snapshot at https://ourfuturewaterintelligence.com/utilities/tokyo-metropolitan-government-bureau-of-waterworks.