110 
this optimum value of about 12 inches (Fig. 4). 
Although sediment yield is influenced by vege- 
tation, temperature, rainfall characteristics, and 
topography, a rough approximation for sedi- 
ment yield can be obtained from the effective 
precipitation. 
Data indicative of the differences between 
the climates of the various drainage basins on 
Kauai and of the supplies of sediments in those 
basins are given in Table 1. These data include: 
( 1 ) a rough calculation of the mean annual 
rainfall for the various drainage areas; (2) ef- 
fective precipitation calculated from rainfall; 
( 3 ) runoff data for the upper portion of some 
of the drainage basins, obtained from gaging 
stations; and (4) the percentage of carbonate 
in the beach sand sample taken nearest the 
stream mouths. 
In general, the table confirms that the great- 
est precipitation and the most runoff occur on 
the windward side of the island where the least 
amount of terrigenous sediment is found in the 
beach sand. The Langbein and Schumm curve 
(Fig. 4) shows that the potential sediment yield 
is appreciably higher when the effective pre- 
cipitation does not exceed about 35 inches, with 
a maximum yield corresponding to a value of 
about 12 inches. The equivalent values in terms 
of annual rainfall on Kauai 6 give the band of 
high sediment yield as occurring where the mean 
annual rainfall does not exceed 46 inches, and 
0 After correcting for evapotranspiration, following 
Langbein et al. (1949: fig. 2). 
FIG. 4. Relation between effective precipitation and 
annual sediment yield (taken from Langbein and 
Schumm, 1958: fig. 3). 
PACIFIC SCIENCE, Vol. XVII, January 1963 
a maximum sediment yield corresponding to a 
mean annual rainfall of 22 inches. Figure 3 
shows that a relatively high percentage of the 
drainage basins on the lee side of the island fall 
within the range of high sediment yield, 
whereas the rainfall in the windward drainage 
basins is appreciably greater and thus produces 
a somewhat lower yield of sediment. The mean 
annual rainfall for each drainage basin and the 
corresponding value for the mean effective pre- 
cipitation are listed in Table 1, columns 4 and 
5, respectively. The Waimea River drainage 
basin (Fig. 3, B), which is the principal basin 
on the lee side of the island, has mean values 
of annual rainfall and effective precipitation of 
70 inches and 53 inches, respectively, which are 
much nearer the value of maximum sediment 
yield than are those for the principal windward 
drainage basins (Fig. 3, E, F, G ) . In fact, much 
of the Waimea drainage basin falls within the 
rainfall range of potentially high sediment 
yield. Also, the sediment yield in the Waimea 
basin is accentuated by the occurrence of ex- 
treme topographic relief in the low rainfall area. 
This combination of factors, tending to increase 
sediment yield, does not occur extensively in 
the windward basins. 
The high carbonate content of the beach sand 
samples on the windward coast tends to con- 
firm the difference in sediment yield between 
the lee and windward drainage basins (Table 1, 
last column). In addition, cursory inspection of 
river banks and river mouths indicated that not 
only is the yield greater on the leeward side 
but also that there is a significant difference in 
the size of the sediment: the leeward streams 
carry large amounts of sand-size material, 
whereas the windward streams carry silt and 
clay-size material. A predominance of silt and 
clay-size sediment indicates chemical weather- 
ing, a process which is enhanced in the wind- 
ward basins by greater vegetation density and 
slower rate of removal of disintegration prod- 
ucts. 
EXPOSURE TO WAVES 
Although no complete statistical wave data 
for spectrums and directions of waves are avail- 
able for the island of Kauai, a general descrip- 
tion of the prevailing wave conditions can be 
