Kawai Nui Marsh was formed at about 400 A.D. The Marsh is actually a big caldera that was formed by the Koolau Mountains. Streams flowed into Kawai Nui Marsh that brought sediment from the mountains. The building up of the sediment made a muddy lagoon floor. Eventually salt-tolerant vegetation began to develop creating a marsh environment near shore. Polynesians used it as a place to fish, plant taro, and raise their cattle. Later, when the immigrants came they used the marsh as a rice patty to plant rice. But now since there are so many houses and buildings around the marsh, there are people that actually use it as a dump.