PHYTOGEOGRAPHY OF MANOA VALLEY, HAWAIIAN ISLANDS 563 
Manoa is situated on the island of Oahu, in the vicinity of the city 
of Honolulu. Residential districts lie along portions of the mouth of 
the valley and lower western slopes. Much of the floor is occupied 
by agricultural lands — taro, bananas, vegetable gardens, etc. Oahu 
is third in size among the Hawaiian islands. It is 46 miles long and 
25 miles wide, with an area of 598 square miles. It is topographically 
distinguished from the other islands by being composed of two elongate 
mountain ranges, Waianae and Koolau. These are of great antiquity, 
deeply eroded, and give evidence of numerous and extensive elevations 
and subsidences. 
The Waianae Range, lying on a NW.-SE. axis, is about 20 miles 
long. Its highest peak, Ka-ala, is 4,030 feet high; this is the highest 
point on the island. The highest point in the archipelago is Mauna 
Kea, on Hawaii, 13,825 feet. The Koolau Range, in which Manoa 
is carved, lies to the northeast of the Waianaes, parallel with the latter, 
at a mean distance of eighteen miles. The Koolau Range is 37 miles 
long, and is the longest range in the archipelago. It is low, its mean 
elevation not exceeding 2,000 feet. The highest peak, Kona-hua-nui, 
rises to 3,105 feet, and lies at the head of Manoa Valley. The range 
is deeply sculptured by subaerial erosion. There are about fifty major 
valleys, with numberless ravines and lateral gullies. Manoa is one 
of the largest of the major valleys. 
Manoa is a well-matured valley, with broad flat floor and slightly 
expanded head. Measured from its mouth or portal (using the 
100 feet contour as a base-line), an airline to the crest of the summit 
ridge is 3.4 miles long. Its width, measured by airline from one 
lateral ridge-crest across to the opposite ridge-crest, varies from 1.2 
miles at the portal to 2.2 miles at the head. Like many other of the 
larger Hawaiian valleys — Kalihi, Kahana, lao, Pelekunu, Halawa, 
Waipio — the head of Manoa is a constantly expanding amphitheater 
of erosion. The valley widens progressively from portal to head, 
at the rate of about 5-6 percent. 
The Koolau Range lies along a NW.-SE. axis. All the valleys, 
of which Manoa is one, that deeply furrow its leeward flanks have a 
dominant southwesterly exposure. The trade winds, which blow 
almost continuously through a major portion of the year, come from 
the northeast. The leeward valleys are thus protected from the trade 
winds by the mountain wall. The maximum of the torrential precipi- 
tation that results from the rising of the moisture-laden trades over 
