The battle happened because Maui's king was elsewhere. Kahekili II, the powerful ruler of Maui, had gone to Oʻahu, leaving the island's defense to his son Kalanikūpule and a cadre of chiefs. Kamehameha I, the ambitious chief of the Island of Hawaiʻi, saw the opportunity. His war fleet sailed across the channel and landed at Kahului, just a few kilometers from the mouth of ʻĪao Valley. An army of roughly twelve hundred warriors advanced inland, led by Kamehameha himself and his war commander Kekuhaupiʻo. The Maui forces took up a defensive position in the narrow valley, where the steep walls should have neutralized any advantage in numbers. For two full days, the armies fought to a standstill. Neither side broke.

On the third day, everything changed. Kamehameha's forces brought forward two cannons — named Lopaka and Kalola — operated by John Young and Isaac Davis, two foreign sailors who had become the king's royal advisors. These were likely the first artillery pieces ever used in warfare between Hawaiian forces, and their effect in the confined valley was devastating. The Maui defenders had no equivalent weapon and no experience against cannon fire echoing off stone walls. Although none of Maui's major chiefs were killed, the rank-and-file losses were catastrophic. Bodies piled into the stream that runs through ʻĪao Valley, choking its flow. Chiefess Kalola and her eleven-year-old granddaughter Keōpūolani managed to escape westward through the valley to Olowalu and then north to Lahaina.

What followed the carnage was pure dynastic calculation. Kalola, having fled with her granddaughter through the blood-soaked valley, offered the young Keōpūolani to Kamehameha as a future wife. It was both a surrender and a strategic move: Keōpūolani carried the highest sacred rank in all of Hawaiʻi, and any children she bore Kamehameha would unite the bloodlines of both islands. The girl who had escaped the damming of the waters would eventually become the mother of Kamehameha II and Kamehameha III, shaping the dynasty that ruled a unified Hawaiʻi. Meanwhile, Keōua Kūʻahuʻula, the last independent chief on the Big Island who had been raiding Kamehameha's territory during his absence, hurried home — setting the stage for further conflicts on Hawaiʻi itself.

Kepaniwai did not end the wars. Kahekili II returned to Maui and acquired his own cannons. In 1791, he tried to invade the Big Island but was defeated in a naval engagement called Kepuwahaʻulaʻula — the Battle of the Red-Mouthed Gun. Kamehameha had to wait for Kahekili's death in 1793 and the civil war that followed before he could finally take Maui, a conquest completed at the Battle of Nuʻuanu on Oʻahu. Today ʻĪao Valley is a state park, green and quiet, its steep walls rising over two thousand feet above the stream. A cannon recovered from the valley floor is displayed at the Bailey House Museum in Wailuku. The water flows clear now. But the name Kepaniwai is still there on the maps, a reminder of the day the stream stopped running.