Juliana Aguilar
California State University Stanislaus

Abstract

Ka’a’awa Valley’s serene and unstudied environment poses many questions. The valley itself holds a main watershed and several tributary watersheds. Since this study is taking place during Oahu’s dry season it has been relatively difficult to observe the water network at its peak. There seems to be only one observable perennial stream, and there is a break in flow towards the middle of the valley. This study focuses on gathering enough information to explain the stream’s break in flow. Geologic, elevation, soils and aquifer data were used to determine which factor played a leading role in the submergence and emergence of water. The research advanced and the answer is very much plausible. All that’s left is to do some more intensive data collection to support the reasoning.

Introduction

As a novice hydro geologic researcher, I found the disappearance of water in the middle of the stream channel fascinating. This observation quickly became the focus of my research question. Groundwater is affected by several factors. The plausible explanations for why water is submerging include topography, geology (which I will focus on), and vegetation. If the dominating factor is topography there should be a trend in the stream’s longitudinal profile depicting a change in elevation were the stream submerges.

If the dominating factor is geology there should be a change in the porosity of the unit directly beneath the submergence of water. Another possible geologic contributor would be a dike causing the water to resurface downstream. To acknowledge vegetation’s role in all of this, we must say it plays a major factor in the storage and usage of water. Much of the water budget I assume is being stored by plants along the riparian corridor as well as through percolation into the soil.

Materials and Methods

Initial data included a 10 meter DEM of the Ka’a’awa Valley which was used to determine the flow of water throughout the valley. Although the DEM served purposeful along the steeper slopes, it characterized the valley as being relatively flat. Being that topography could possibly serve as a major contributing factor to the submergence of water along the main stream channel, the DEM was reclassified to focus on this area.

Data points were collected using a Trimble along the main stream channel in the center of the Ka’a’awa Valley. Temperature, flow, and elevation measurements were calculated along several of these points. There was an attempt to correct the Z values along the main stream channel to better focus on what was happening in the topography, unfortunately the Trimble unit we were using served to be unreliable. With thick canopy surrounding the stream it was difficult to obtain vertical precision less than 3% with a few outliers as high as 15%. About 1/3 of the points had to be thrown out. After importing the points collected on the Trimble into Pathfinder, they were moved into ArcMap 10.1 and analysis began.

Aquifer, soils, and geology layers were also brought into ArcMap and overlayed with points that had been collected to try to find any correlations between them. These layers would also explain why the water was submerging and emerging at the allocated points.

Analysis of this data was performed using ArcGIS 10.1.

Results

Figure 1 shows the main water channel’s longitudinal profile. We can see that there are no major hiccups in Ka’a’awa Valley’s topography. There are a few outliers shown which could be caused by the lack of vertical precision along the stream corridor (most likely due to the dense vegetative cover.)

Figure 1 This stream profile shows us the elevation changes relative to the distance from ‘Atlantis’ (a movie set found towards the back of the Ka’a’awa Valley.)

Figure 2 shows us areas of differing soil types. We can see a multitude of soils among the Ka’a’awa Valley but no major change along the stream channel.

Figure 2 Here we see the different soil types near mouth of the Ka’a’awa Valley as well as this study’s focus section of the stream. The blue lines represent wet areas and the brown line represent the dry.

Figure 3 presents us with a map displaying the different types of geologic units found in the Ka’a’awa Valley. Holocene quaternary alluvium (younger) and Pleistocene quaternary alluvium (older) dominate the riparian corridor.

Figure 3 Depicted in this image are, the white, the younger alluvium, and the yellow older alluvium.

Discussion

If the topography was responsible for the water disappearance the distance between the two red bars in figure 1 would show a climbing trend. Since water cannot flow up, it would sink underground and come back up were the topography leveled out. The best fit line supports the notion that the topography is descending at a regular interval.

Initial theories focused on water percolation rates and water capacity within these soils. This idea was quickly discounted; primarily because the map shows little to no change in soil type along the riparian corridor. Also, soil’s data is typically taken at shallow depths and could not account for the water’s ground immersion.

The next step was to look into aquifer data, the metadata was detailed enough to imply that there were a handful of dikes in the area of study. If there was a dike hidden underground, it could possibly cause the water to surface were it does. Figure 4 shows the possible location of a dike. Ground truthing served to be a purposeful exercise because a dike was nowhere to be found. After looking into the geologic data more closely the possibility that the geologic map was slightly incorrect became more plausible. If the younger alluvium was actually thinner than the map showed, and the older more impermeable alluvium actually stretched to the area were the water reemerges, we’d have a definitive answer.  Figure 5 shows a possible rendition of where the younger alluvium would need to be in order for the water to reemerge.

Figure 4 shows the three major aquifers within the Ka’a’awa Valley. The metatdata suggests there should be a dike in each aquifer.

Figure 5 the map featured on the left shows the geologic units in the areas they are thought to be. The map featured on the left is my own rendition of where the young alluvium would need to be in order for the water to reemerge.

Future Work

The water channel mystery is just about over. We have one commercial left until the villain is unveiled and this case can be closed. If given more time to collect data I would suggest collecting hand samples to verify the old alluvium expands to a greater area than the map suggests. Also, if GPR could penetrate deep enough we could possible find a contact point along the area where the water resurfaces.