During the first few days of my visit to Hawaii, Brendan and I stopped by an overlook of Pololu Valley on the wet side of the Kohala Peninsula:
We talked about backpacking up above the valley for a better view of the 1000+ foot headwalls later on in the field season that day. This weekend, we got the opportunity to do just that. We began our trek into the jungle by driving across the green pastures of Kahua Ranch on the Kohala Peninsula. Our goal was to hike to the heads of Pololu and Honokane Nui Valleys via trails that followed water supply pipelines and drainage ditches for the ranches in the area.
We owe the opportunity for land access to Charles Cosgrove of Kahua Ranch and Pono Von Holt of Ponoholo Ranch. Charles was kind enough to give us the keys to the ranch so that we could drive through locked gates to our trailhead, where pasture meets jungle:
Although there was a cleared path for the water pipeline to follow, it was well overgrown by tall grasses that stretched above our heads. A few overflow valves created some particularly boggy conditions that made me glad I’d brought my muck boots. Some sections were almost waist-deep, as I found out from experience. The swampy conditions are not just caused by overflow valves — they are a consequence of clay-rich soils that trap water at the surface.
The trail weaved us between the jungle and boggy terrain parallel to the waterline. We waded through the grass and mud with caution — we wanted to avoid any encounters with the aggressive wild boars that populate the area. We were extra cautious after hearing Charles recount a story of a boar that headbutted his car on his way home the previous night. We kept a lively conversation going to warn any pigs of our approach that might be resting in our path.
After slogging through the mud along the pipeline trail and taking a detour into the jungle, we met the Kehena Ditch trail. The trail once provided access to workers who built the irrigation ditch in the early 1900s to divert water from drainages to nearby ranchland. The ditch trail is seldom used by anyone today — mostly boar hunters and ranchers. And geologists.
After a few hours of hiking, we crossed into open Ponoholo pastures and were greeted with a spectacular view of Honokane Nui Valley. Tourists in Hawaii often spend several hundred dollars for a quick helicopter tour of these valleys, but we had the opportunity to stand there and appreciate the 2000 feet of relief below us, at the edge of the pasture.
The scientific goal of our trip was to observe the styles of erosion that shaped these natural amphitheaters. In map view, they are U-shaped. Typically, valleys form a V-shape in map view, so the situation at Pololu and Honokane Nui is somewhat unique. The typical V-shape drainage has a gradual slope leading up to the head of the drainage, rather than sharp drop.
Two processes have been attributed to the formation of these valleys. In the 1980s, it was argued that they were a result of groundwater sapping. Groundwater sapping occurs when water flows through rocks in the subsurface and weakens them, thereby increasing the erodibility of the rock. The sapping occurs at the base of the cliffs and undermines the strength of the overlying wall of rock, causing the cliffside to collapse. More recently, it has been argued that waterfall erosion is a major player in the formation of the valleys. Brendan and I got a boots-on-the-ground perspective to make our own observations of the erosional processes in the valleys.
After snapping a few photos, we headed to the Ponoholo Ditch Cabin, where we would basecamp for the two nights we spent in the area. [Huge thanks to Pono for allowing us to use the cabin!]
We dropped our gear and headed back out to the overlook for the rest of the afternoon. We took some time to appreciate the views, and hiked back to the cabin for the night.
The next day, we hiked even farther makai (seaward) and went to the head of Pololu Valley. It’s not as deep as Honokane, but impressive nonetheless:
We returned to the cabin once again to rest up before our hike out the next morning. Big thanks again to the folks at Kahua, Parker, and Ponoholo Ranches for giving us VIP access to some of the most badass geology on the big island! We took the rest of that Sunday off and went up to Mauna Kea for a VIP tour of the Subaru Telescope – more on that in my next blog post.
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