Manoa Falls, HI

In our first journal, I wrote about my love for Mānoa Falls. It is an hour-long hike that leads to a waterfall. And like most waterfalls in Hawaiian mountains, if you ascend 100 feet above the waterfall, you arrive at another waterfall directly above it, feeding into the one below. However, at the same time, I do not understand many of the processes at the work that result in me experiencing these things. A couple questions arise. How did these waterfalls form? Does it have to do with the rivers? Is it similar to the way that cliffs are formed? Where does the water come from?

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Manoa Falls flanked by green tropical growth with rocks and danger sign in foreground

Attempting to connect it to the broader themes and processes we learned this year, I assume that the water from the river flowed until it reached a drop off point. That hard rock layer at the top was layed on top of softer rock or materials below. Eventually due to the water and formation of a plunge pool, the falling water or eroding rock material wear away at the softer rock material below. Like a cliff, the hard rock is undercut by the erosion of the softer material below and as the base structure of that waterfall moves back, the hard rock on the top is not supported and erodes or falls down as big boulders. And as a result, the waterfall recedes further back. That can be seen here with water pouring over at a high velocity and discharge, creating a plunge pool, and at the very bottom surrounded by big boulders possible transported from the top of the waterfall or as a result of the rock material in the waterfall eroding when the soft rock at the base concaved in.

This begets the question, where did the water come from? Unfortunately, I am forgetful and not entirely understanding of these processes, but I will try to deconstruct and understand them bit by bit from my own knowledge. Manoa is located in the central part of the island and is rainforest. Hawai’i is an island in the middle of the pacific roughly 20° N. Returning back to the concepts of the coriolis effect, latent heat transfer, and the hydrogen cycle, very warm air is being transported from the oceans through northeasterly trade winds. Through the orographic precipitation, as humid water encounters a mountain, I would expect the air to rise. As the air rises in elevation, it becomes cooler and denser as a factor of temperature change with elevation. The humid air performs condensation and then precipitates rain fall on the windward side of the mountain. However, the rest of that air, now cool and dry, goes over the mountain and settles back down to earth on the other side of the mountain. This would be the leeward side of the lands, which are much drier during the summers than the windward side. Returning to orographic precipitation, the winds hitting the East Range will be directly from the huge body of water adjacent to the range, resulting in humid high pressure wind hitting the mountains, rising in elevation, becoming more dense and raining. As we covered in HW #2, this plays a major role in why the leeward sides are much greener and receives more rain, on average, than the windward sides of the island. Since Mānoa is more in central, closer to the categorized leeward sides, it makes sense that this area will receive a lot of rain. This results in the ecology of the place being so densely packed with plants and a lots of coverage, flowing rivers, and waterfalls. I would hypothesize that the rain is a major factor of there being flowing rivers and waterfalls in Mānoa. And by mere observation, I notice that the waterflow is flowing with much more discharge, volume, and velocity on rainy days than on dry days.

I was interested in learning the vital role water and rivers play in our experienced topographical regions. The water is what carves up the place leading to valleys or waterfalls or other peculiar structures. Never before had I considered the importance of rivers. Right now, I live in Pālolo valley with the Wai’ōma’o stream flowing behind my house. My grandfather lives a few blocks up from my very house. For the literal past two years, he has been working on leveling the house because the house was slanting. My great grandmother moved to that house along with her four children in the early 50s or 60s. At the time, it may have looked like a great investment. An elevated house with a gorgeous yard sitting a top a hill overlooking the Pālolo stream. However, with my knowledge of geological processes, I know better now. With many houses in my valley, they are situated directly on top of the valley face or on the side of the mountain basically. The state has been doing construction on my street for the last three months straight fixing the shifting road. This goes hand-in-hand with my grandfather, and even my house in the future. For the last two years he has been hiring contractors to relevel and reinforce the house because it sits on the top of a “V” where the river runs. I know now from our virtual labs that rivers experience erosion at the cut bank and deposits of sediment at the point bar. Over time, this results in the physical movement of the river and all of the land features surrounding it. Eventually that river will continue to erode away at that cliff side. For my grandfather, the side of the house nearest the river was sinking or dipping down. 60 years ago this was no issue, but now many years later and water cycles exacerbated by climate change, erosion poses a threat. Learning about geology has been so useful in providing me with knowledge and terms to explain what I have always experienced in my life, but lacked the words to describe. It also taught me never to buy a house near a river on the levees or outer edges because as it moves, my house will too inevitably be affected.

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Tropical mountain tops in clouds

Here is a picture of the rain clouds accumulating on the mountain tops on a crater hike behind my house.

 

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