Hāmākua Hawai`i, Uncategorized

Notes on the Geology of Waipi`o Valley

 

Elsewhere we have briefly discussed the role of water in shaping Waipi`o Valley, mentioning the five major drainages that contribute to the Valley’s form and stream flow.  Hi`ilawe, the most famous, is also the closest to the mouth of Waipi`o.  Waima goes nearly straight back from the Valley’s coastline toward Waimea, taking water from the slopes of the old cinder cones called Pu`u Lala (or Alala, on some maps) and Pu`u Ka`ala in a northeasterly direction to the sea.  On the Waimea side of the slopes of  Pu`u Ka`ala are subdivisions, including the one called Pu`u Nani.

The Waima axis of the Valley may have been formed as water flowed down the overlapping lava fields of Kohala and Mauna Kea, but toward the back of Waima the three other drainages — Koiawe, Alakahi, and Kawainui —  have followed a fault zone running on a southwest-northeast axis.  The cracks and zone of weakness from Kohala’s summit provided a path for streams to form that now flow almost at a right angle into the channel carved by Waima.  Before the gulches got too deep (about 1,200 feet above sea level, to judge from topographic maps), Kawainui stream was the headwater of Waimanu.  The faults in the underlying rock, eroding more rapidly, in part because of the erosion of the Alakahi stream, captured Waimanu’s original water, leaving a “wind gap,” a v-shaped notch through which Kawainui used to flow, abandoned now to the breezes.   Waimanu, robbed of the water that used to flush through in powerful quantities, is slowly filling with debris in its rearward regions, since the slides and slumps from its walls are not washed down and out of the Valley.

In times long before humans saw it, Waipi`o (and the other flat-bottomed valleys of the windward coast) was both narrower and deeper.  As water cuts its way into mountain slopes, the stream beds tend to be only as wide as floods will cut them.  Waipi`o Valley is obviously many times wider than its river, but the reason is not at all obvious.  The most recent ice age ended some 10,000 years ago.  As the great glaciers and polar ice caps began to melt at the beginning of the warming trend which we humans are still enjoying (temporary though it is), the ocean began to rise.  The valleys of Hawai`i that had carved the lava down to sea level started to have sea water flooding into their mouths.  The rocks, gravel, sand and dirt carried by stream erosion began to be halted before getting to the former shoreline, filling in the old stream bed and raising it so that the erosion-produced debris began to back up behind it.  Eventually, the wide floor of Waipi`o was filled with this material (what geologists call “alluvium”) to a depth, at the mouth of the Valley, of as much as 300 feet.

Most of the major valleys on the windward side have submarine canyons that extend on into the deep.  Waipi`o does not.  Some suggest that it did at one time have such a feature, created by stream-flow when the shoreline was lower, but that it was filled with alluvium.

In some traditional Hawaiian stories,  it was Kamapua`a, the demigod related to Lono, who, in his form as a giant pig, ripped the valleys out of the mountain slopes in his passionate search for food.  Either way, the story behind the scenery of Waipi`o and its companion valleys on the windward slopes of Hamakua and Kohala is amazing.

 

Copyright 2003  by Hugh R. Montgomery

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