Before European contact

By 1750, Kawainui had been developed by the native Hawaiians into a 400-acre (160 ha) fishpond used for food. Common fish included mullet, awa, and oʻopu. Irrigated loʻi kalo around the edges of the fishpond, as well as nearby patches of dryland kalo, banana, sugarcane, and sweet potato, served as an additional food source. There were also at least three significant heiau built around the fishpond,^[6] including Ulupō.^[7] Some evidence suggests that the Hawaiians also improved the channel connecting Kawainui to Kaʻelepulu.^[8]

After European contact

In 1778, James Cook arrived in Hawaii, marking the first contact between Hawaiians and Europeans. European arrivals carried diseases, including measles, smallpox, and influenza, to which the native Hawaiians had no prior exposure and no immunity; the total population of Hawaiians dropped from roughly 300,000 to 50,000 within 50 years. This made large-scale work such as maintenance of fishponds impossible, causing Kawainui Fishpond and the surrounding agricultural area to become unused and overgrown.^[9]

In 1848, the Great Māhele and the growing economic importance of Honolulu drove much of the native Hawaiian population away from Kawainui and the surrounding area.^[10] The concept of private land ownership was completely unknown to most Hawaiians, who therefore did not file claims for the land they were entitled to.^[11] The vast majority of property in and near Kawainui therefore went to aliʻi, especially Kalama. Crops including ʻawa, wauke, and sweet potatoes continued to be grown in the area at this time. Cultivatable areas of the valley floor around the fishpond, which were gradually becoming a marsh, were used as a site for loʻi kalo.^[12]

After Kalama died in 1870, her land was sold to American lawyer Charles Coffin Harris, including Kawainui Fishpond which was still intact at the time.^[13] Beginning in 1878, water that would have flowed into Kawainui from Maunawili was instead diverted to Waimānalo for irrigation of sugarcane.^[14] After Harrisʻ death in 1881, his daughter and heir Nannie Roberta Harris owned the ahupuaʻa of Kailua until 1917.^[13]

In the 1900s

While under Harris family ownership, Kawainui was used for rice cultivation by Chinese farmers; the vast majority of the fishpond was being used for this purpose by 1900, but it was still publicly known as a fishpond.^[15] In 1917, Nannie Harris sold her land to Harold Kainalu Long Castle.^[13] In the early 1920s, water from Kawainui was drained and used for sugarcane in Waimānalo.^[14] Rice cultivation began to decline, and most of the former rice paddies were used for ranching^[16] by Harold Castle, who introduced large numbers of invasive plants in order to feed his cattle.^[10]

During World War II, Kawainui was used as a training area by the United States Armed Forces, which leased it from Kaneohe Ranch.^[16]

In the 1950s, the Honolulu Construction and Dredging Company ran a rock crusher along one end of the marsh. The City and County of Honolulu subsequently leased that area and used it as a site to burn trash until 1962.^[17]

In 1956, Kaneohe Ranch pumped water out of Kawainui until the water table dropped by almost four feet, making the area more usable for grazing as part of a land reclamation process that was discontinued in 1965.^[18]

The Hawaii State Board of Geographic Names officially designated Kawainui a marsh in 1982. In the early 1900s, it had been inaccurately described as a swamp.^[6]

In the 2000s

In February 2005, Kawainui was designated a Ramsar Wetland with site number 1460.^[1]

As of 2019, a mass of peat about four feet thick was covering much of Kawainui. Trees were beginning to grow in it, and papyrus had been seen growing as well. Multiple nonprofit organizations were working to restore areas of the marsh.^[10]