Kawai Nui literally means "the great water" and was once an area that supported a large native Hawaiian settlement. Hundreds of acres were devoted to fishponds and taro cultivation with one of the most extensive continuous terrace areas on Oahu. The number of heiau or sacred places of worship are evidence of the importance of the area. Although much smaller in size today it still remains the largest fresh-water marsh in the state. Designated a Ramsar Site it becomes part of a treaty for the conservation and sustainable utilization of wetlands.

Kawai Nui Subwatershed is the largest of the subwatersheds in Kailua and covers approximately 7946 acres or 68% of the total Kailua Watershed. It also includes the most extensive streams and water bodies where surface water flows to Kawai Nui Marsh and out through Oneawa Canal. The storm drain system carries water from residential, agricultural (crop and livestock) and commercial areas. It has the lowest percentage of impervious surface cover. Kawai Nui Marsh is a wetland area that covers 1041 acres within the subwatershed and is a habitat for endangered waterbird species. Preservation land use zones cover 67% uopn which an operational quarry and three closed landfills are located. This watershed empties into Kailua Bay through Oneawa Canal.