It is roughly circular in shape and was formed by a single shield volcano. Kauai is the fourth largest of the Main Hawaiian Islands. Geology and ground-water resources are described by MacDonald et al.
Mountainous terrain occupies the north, west and central part of the island.
The gentle east and south slopes are cut by shallow gulches. Adjacent to the coastal plain are the longest stretches of excellent beach to be found in the state. The major streams of Kauai have their headwaters in the large central caldera of the volcano. The low slopes, deep soil, high rainfall and tall vegetation enhance stream permanence on Kauai. On the east side of the island permanent streams are fed by springs stemming from the dike complex within the volcano.