Geomorphic Contrasts in Koolaus — PALMER 
from a fairly smooth upland surface. The map 
of an area of five square miles southeast of 
Wahiawa (Fig. 8) shows part of the extensive, 
smooth upland used for pineapples, with 
Waikakalaua Gulch and the South Fork of 
Kaukonahua Gulch incised for more than a 
Fig. 7. Mean annual rainfall in inches on Oahu. 
(After the Territorial Planning Board, First Progress 
Report, pi. 52, p. 116. Feb. 1929.) 
hundred feet. Here the upland slopes west or 
southwest about 200 feet to the mile, or 
about 2°. 
In the northwestern part of the range the 
streams .are much longer than those in the 
southeastern part, which operates in two ways 
to let only narrow gulches be cut. For one 
thing, the longer courses cause gentler gradi- 
ents and thus less velocity and less erosive 
power. And also, the longer courses require 
the removal of a larger total volume for a 
given depth and width of valley, so that in a 
given time only narrower and shallower val- 
leys will be cut than by shorter and steeper 
streams. All this is in addition to the present 
hypothesis that these streams have been at 
work for decidedly less time than the streams 
in the southeastern part of the range. 
COURSES OF THE LEEWARD STREAMS 
Comparison of two maps of the stream 
courses in areas, of about five square miles 
each, reveals marked contrasts between the 
ends of the Koolau Range (Fig. 9a, b). The 
Palolo-Waialae area, east of Manoa Valley, 
309 
is characteristic of the southeastern end and 
the Kamananui area is characteristic of the 
northwestern end of the Koolau Range. 
Where streams appear to have operated for 
a much longer time, east of Manoa, they have 
been able to eliminate much of their original 
windings in the process of cutting more and 
more direct courses to the sea. Their original 
steep gradients, down the flank of the highest 
part of the range, gave them greater speed 
and erosional energy that would favor straight- 
ening their courses. 
In contrast, the streams from Kaiwikoele 
to Kaunala, of the northwestern Kamananui 
area, have much more winding courses. They 
began their work on less steep slopes and were 
therefore less strongly driven by gravity and 
meander more. Perhaps the gentle gradient 
even favors lateral swinging of the streams 
and meandering. When originally built the 
surface of a lava shield is far from smooth. 
Successive lava flows have steep edges and 
do not overlap perfectly. Their edges are 
Fig. 8. Map of a smooth area of 5 square milea 
southeast of Wahiawa, with sharply incised gulches 
Contour interval 100 feet. 
wavy, and these cause the streams to follow 
the irregularly winding low places between 
flows and thus to have winding courses at the 
start. It also appears that these streams have 
been at work for less time, and thus have had 
less time to straighten their courses. 
