The Wailuku River is a 28.0-mile-long (45.1 km) water course on the Island of Hawaiʻi in the Hawaiian Islands. It is the longest river in Hawai'i and its course lies mostly along the divide between the lava flows of Mauna Kea and those of Mauna Loa to the south. It arises at about the 10,800 feet (3,300 m) elevation along the eastern slope of Mauna Kea (19°48′26″N155°25′13″W).