The U.S. Geological Survey conducted a field study from March to October 2011 to identify ditch characteristics and quantify seepage losses and gains in the East Maui Irrigation (EMI) diversion system, east Maui, Hawaiʻi. The EMI diversion system begins at Makapipi Stream in the east and ends at Māliko Gulch in the west. It consists of four primary ditches known as the Wailoa, New Hāmākua, Lowrie, and Haʻikū Ditches. Additional ditches that connect to the four primary ditches include the Koʻolau, Spreckels, Kauhikoa, Spreckels at Pāpaʻaʻea, Manuel Luis, and Center Ditches. Ditch characteristics for about 63 miles of the EMI diversion system, excluding abandoned ditches and stream conveyances, were identified. About 46 miles (73 percent) of the surveyed diversion system are tunnels and 17 miles are open ditches—in which 11 miles are unlined, 3.5 miles are lined, and 2.5 miles are partially lined. The Wailoa, Kauhikoa, and Haʻikū Ditches have greater than 96 percent of their total lengths as tunnels, whereas more than half of the Lowrie Ditch and Spreckels Ditch at Pāpaʻaʻea are open ditches. About 70 percent of the total length of lined open ditches in the EMI diversion system is located along the Koʻolau Ditch, whereas about 67 percent of the total length of unlined open ditches in the diversion system is located along the Lowrie Ditch. Less than 4 percent of the EMI diversion system is partially lined open ditches, and about half of the total partially lined open-ditch length is in the Spreckels Ditch. EMI regularly maintains and repairs the diversion system; therefore, ditch characteristics documented in this report are representative of conditions existing during the period of this study. Discharge measurements were made along 26 seepage-run measurement reaches that are a total of about 15 miles in length. The seepage-run measurement reaches represent 23 percent of the total length of ditches in the EMI diversion system. Discharge measurements were made along the measurement reaches during periods of stable ditch flow in the months of June, August, and September 2011. The discharge measurements indicate that Koʻolau Ditch and Spreckels Ditch at Pāpaʻaʻea generally had seepage losses, whereas Wailoa, Kauhikoa, and New Hāmākua Ditches had seepage gains within the measured reaches. The Manuel Luis, Center, Lowrie, and Haʻikū Ditches had variable seepage losses and gains within the seepage-run measurement reaches. Open-ditch measurement reaches generally had seepage losses that ranged from 0.1 cubic feet per second per mile of ditch at the Lowrie Ditch to 3.0 cubic feet per second per mile at the Koʻolau Ditch. Tunnel measurement reaches generally had seepage gains that ranged from 0.1 cubic feet per second per mile at the Manuel Luis Ditch to 5.2 cubic feet per second per mile at the Wailoa Ditch.