Figure 1

Figure 2

Note the contrasting morphological styles of each coast reflecting major growth and mass-wasting processes characteristic of the Hawaiian Islands. The morphology of the region between Port Allen and Makahu�ena Point on the southwest flank of Kaua�i is the upper portion of a gigantic submarine debris avalanche. The insular shelf and slope collapsed catastrophically and was transported downslope. Note the large blocks and associated debris that forms a cone-shaped deposit in the central part of the image.

The seafloor morphology of the upper southeast margin of Kaua�i between Makahu�ena Point and Nawiliwili is composed of a relatively broad, gentle, southeasterly sloping terrace. The terrace is 10-13 km wide and trends northeasterly about 45 km. This terrace was constructed by lavas during a sea level low stand. Note the abrupt change of slope at the seaward edge of the terrace, which marks the position of the shoreline when the terrace was created. The steep change in slope at the outer terrace reflects the increased slope angle that occurs when lava passes from land to sea. The terrace was subsequently submerged as sea level rose concomitantly with island subsidence.

Figure 3

The backscatter data from the southwest margin between Makahu'ena Point and Port Allen shows that the debris avalanche blocks and associated deposits are characterized by high backscatter. The debris avalanche deposits form three high-backscatter features about 10 km long by 4 km wide each that trend nearly north-south in the central portion of the southwest flank. The remainder of the image surrounding the avalanche debris is characterized by a complex, high-backscatter, dendritic pattern. These high-backscatter dendritic features probably represent pathways (gullies, chutes, or canyons) for downslope sediment transport.

Backscatter data from the southeast margin of Kaua'i between Makahu'ena Point and Nawiliwili shows that the steep seaward edge of the submerged terrace is incised with high-backscatter gullies and canyons that coalesce downslope into broad, high-backscatter deposits. The broad areas of high backscatter on the lower slope correspond to the broader, smooth, lower-slope sediment mantle visible in the shaded-relief map.

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Figure 21 (A)

Figure 21 (B)

Note that some of the high-backscatter features shown in 21B are mounds with a relief of 1 - 3 m.

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