The East Maui Irrigation System is Hawaii's most dramatic water story. It began with the construction of the Old Hamakua Ditch built between 1876 and 1878. This privately financed, constructed and managed irrigation system was one of the largest in the United States. It eventually included 50 miles of tunnels; 24 miles of open ditches, inverted siphons and flumes; incorporates approximately 400 intakes and 8 reservoirs. The aqueducts bring water from steep tropical forest dissected by many ravines on the Windward and wet slopes of Haleakala, a 10,000 foot high dormant volcano, to the fertile semi-arid central Maui isthmus. They provide half the irrigation water to the sugar growing area of Maui.
This irrigation system demonstrated the feasibility of transporting water from steep tropical forested watersheds with high rainfall across difficult terrain to fertile and dry plains. Over the years of the development of this system, many engineers gained experience in building irrigation systems. They used what they learned from the East Maui Irrigation System to develop irrigation systems for the Western United States. For instance, engineer M.M. O'Shaughnessey, in charge of constructing the Koolau Ditch in 1904 and 1905, subsequently built the San Francisco Hetch Hetchy water system.
- Old Hamakua Ditch took two years (1876-1878) to complete at a construction cost of $80,000.
- The completed Old Hamakua Ditch was 17 miles long and had a capacity of 60 million gallons per day.
- The second ditch was the Spreckles Ditch. When completed at a construction cost of $500,000, it was 30 miles long with a capacity of 60 million gallons per day.
- The sugar companies continued to build irrigation ditches. Before World War I, the New Hamakua, Koolau, New Haiku, and Kauhikoa ditches were built.
- The largest ditch, the Wailoa Ditch, was completed in 1923.