Kohala (mountain) - Hydrology

Hydrology

The windward side of the Kohala mountains is dissected by multiple, deeply eroded stream valleys in a southwest-northeast alignment, cutting into the flanks of the volcano. North of Kohala's summit the volcano's northwest-southeast trending rift zone separates rainfall into two streams, going southeast, into Waipiʻo Valley, or northwest, into Honokane Nui Valley. When the volcano was still active, vertical sheets of magma, arranged in what is known as dikes, forced their way out of the magma reservoir and intruded into the rift zone, which was weakened by the collapse. As the dikes forced their way up, they formed fractures and faults parallel to the rift zone. The exertion caused by the dikes produced a series of faults along its length, forming horsts and grabens (fault blocks). Northeast of the Kohala summit, where the most rainfall occurs, the faulted structure prevents summit rainwater from naturally flowing northeast down the mountain slope. Instead, the rainwater flows down laterally and empties in to the back of what have thus become the largest valleys (Waipiʻo and Honokane Nui).

The northwestern slope of Kohala has few stream valleys cut into it, the result of the rain shadow effect, as the dominant trade winds bring most of the rainfall to the northeastern slope of the volcano. The seven windward valleys of Kohala are, from southeast to northwest, Waipiʻo, Waimanu, Honopue, Honokea, Honokane Iki, Honokane Nui, and Pololu. The size of the valleys reflects the amount of drainage they have been able to capture; Waipiʻo, the largest, at some point drained Waimanu Valley as well, and the former headwaters of Waimanu are now one of its branches. Waimanu valley, cut off from its headwaters to the summit and left with Wai'ilikahi as its main tributary, still had managed to form a substantial floodplain, although less than half the size of Waipiʻo. Honokane Nui, the third largest valley, has its drainage almost exclusively from the north side of the summit, and as a result is a deep, long, narrow bottomed valley with little habitable floodplain.

The remaining valleys derive all their water from the lower northeastern slopes of the volcano, and as a result are smaller. In addition, there are innumerable gulches of various depths which have not yet reached sea level and began to form flood plains, becoming valleys. some of these are more than 300 m (984 ft) in depth. The depth of the actual valley walls at the seashore range from 300 m (984 ft) at the Waipiʻo Valley Lookout at the southeastern corner of Kohala mountain, to a high of over 500 m (1,640 ft) in Honopue, then diminishing to less than 150 m (492 ft) at Pololu Lookout at the northern end of the valleys. In the back, deeper in the mountain, all of the valleys have sheer walls well in excess of 1,000 m (3,281 ft). Numerous spectacular waterfalls cascade or fall over these cliffs, some, such as Hi'ilawe falls in Waipi'o valley, have single drops exceeding 300 m (984 ft), making them some of the world's highest waterfalls.

The volcano stayed active well into the formation of these mountainside valleys, as illustrated by later Pololu lava flows, which covered the north and northwestern end of Kohala volcano and often flowed into Pololu Valley. Recent seafloor mapping seems to show that the valley extends a short way beyond the seashore, then terminates at what may be the headwall of the great landlide found off the northeast coast of Kohala.

In addition to being a primary factor in the development of the largest valleys, the dike complex plays another important role —that of creating and maintaining the water table. Hawaiian lava is extremely permeable and porous. Rainwater easily seeps into the lava, creating a large lens of fresh water below the island surface. In most places, the top of the water table is just a few feet from sea level, as the freash water lens is floating on salt water. Unlike porous lava flows, however, dikes cool underground into dense rock with few cracks and vesicles. The dikes act as impermeable vertical walls through which groundwater cannot flow. Deep layers of ash compact into impermeable layers between lava flows, preventing percolation of the rainwater. Rainwater that would ordinarily seep right through the ground gets trapped in the dike "reservoirs," located well above sea level. The groundwater is confined to a flow along the dike until it finds a means to escape.

In Kohala, the numerous dikes near the summit inhibit groundwater from seeping downslope to the northeast, where it naturally wants to go. Rather, the Kohala dike complex guides it northwest or southeast, down the axis of the rift zones, just like the surface water. On the other hand, the three smaller valleys between the large ones - Honopue, Honokea, and Honokane Iki - as well as the many smaller gulches which are not yet valleys, are deprived of groundwater by the orientation of the rift zone and its dikes. Without the large amount of water that is received by the bigger valleys, these valleys grow far more slowly. Due to its topography as essentially a flat crater floor surrounded by cones and fault scarps, the main caldera is affected relatively little by erosion from water.

Most of Kohala's summit groundwater ends up in either Waipi'o or Honokane Nui; the enormous amount of water routed through these valleys results in a large amount of water erosion, which causes the valley walls to frequently collapse, accelerating valley development. This abundance of surface water inspired the first of Kohala's three massive irrigation diversion "ditches", the Upper Hamakua Ditch, although the water was initially used for transporting ("fluming") sugarcane to the mills rather than irrigation. Surface water supplies, however, were not stable enough for the large scale plantation economy developing in Hamakua and North Kohala, so later ditches tapped the spring fed base flows of the valleys and larger gulches. Only Waimanu Valley escaped with none of its tibutaries tapped, along with the 16 smaller streams between Waikoloa Stream and the Wailoa river in Waipi'o Valley.

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