Are your rocks better than average?

[Note: read previous articles about this project for important background details if you haven’t already.]

A dry, desert wash can turn into a raging river within minutes. We are mostly accustomed to seeing dry canyons with motionless piles of boulders and cobbles strewn about. But, with enough water, truck-sized boulders can be sent rolling through desert canyons during a good rainstorm.

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Truck…or truck-sized boulder?

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Brendan stands at the end of a reach of this channel that has been scoured out by a recent flooding event. The “bathtub ring” in the foreground represents the bottom of the channel prior to scouring — before the flood, a pile of cobbles rested below this “bathtub ring”.

Flow events like the one in this video are what shape the canyons and washes that we find in the arid southwest or on the dry side of Hawaii’s Kohala Peninsula. Flow occurs periodically in arid environments and, over thousands of years, cuts channels into the landscape. In places like Utah and Arizona, “slot canyons” are a common manifestation of bedrock erosion:

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Northeast Spur Canyon, located in southeast Utah. Water and sediment together carved this slot canyon over time. If you look closely, you can see evidence for previous flow levels on the canyon walls.

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A less claustrophobic reach of Northeast Spur Canyon in Utah…Sand, gravel and cobbles rest on the streambed until running water puts them to work eroding the bedrock.

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A deeply incised canyon on the dry side of Hawaii. The layering in the canyon is a stack of basalt flows that has now been exposed by erosion. Each flow is about a meter thick.

Sediment particles that are transported by both large and small volumes of water act as tools to carve canyons into bedrock. Like any seasoned tool in your arsenal, the rocks that act as “tools” display wear and tear. A cobble or boulder that is well rounded has traveled farther down the channel than one that has angular, sharp edges. That is, the rough edges are scraped off as the rock moves downstream.

One of the less glamorous tasks that we carried out in the Kohala channels is called a “point count”. During this exercise, Brendan and I measured and described the roundness of 300 randomly selected rocks (per location) to characterize the sediment in the channel of interest. This activity provides important information about what kind of “tools” we are working with for each channel.

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The Powers Index of Roundness ranks sediment from 1 to 6: 1 being the most angular and 6 being the most well rounded.

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Brendan measures the width of a boulder in Waianaia Channel on the wet side.

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Sometimes, during a point count, you find cool stuff: like this weathered peridotite xenolith. This fragment of the earth’s mantle was caught up in lava during an eruption. The xenolith came from at least 14 km (the thickness of the earth’s crust below Kohala) below the earth’s surface via volcanic vents.

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Another (less weathered) peridotite xenolith.

Brendan and I were fortunate enough to be in Hawaii during Tropical Storm Flossie. Flossie is just the type of event that brings enough rain to an arid landscape to induce flow in channels. Brendan and I both got STOKED about the prospect of catching a flash flood in the act because, earlier in the season, we were measuring the sediment transport distance following an earlier flooding event that ripped through this dry, desert stream channel.

So, how do you measure how far sediment has moved after a flash flood?

By using “better than average rocks” (BETA)! A BETA rock is a concrete cobble of a known volume and density that contains a readable Radio Frequency Identification (RFID) chip. BETA rocks are deployed in piles of 40-75 (sometimes more) units in the stream channel that can later be found by an RFID reader!

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A concrete BETA rock, dyed blue. An RFID tag is embedded inside the BETA rock. Don’t eat this.

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Brendan scans the streambed with the RFID reader. I like to refer to it as “the minesweeper.”

Each time the minesweeper goes across an RFID rock, it beeps and displays a unique number that is recorded in our GPS unit, along with the distance the BETA rock has traveled.

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Brendan goes to great lengths to make sure the whole streambed is scanned…

BETA rocks were distributed by collaborators last summer in various locations across the two channels we studied in Hawaii. In May of this year, there was a large flooding event that moved the BETA rocks from where they were placed last year. We weren’t there to see it, but we observed the evidence in other ways. One lucky BETA rock traveled about 500 meters down the channel from the deployment site in the May flooding event!

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Brendan holds his hand up to the level of the water during the last flow event in Puanui Channel. The water level is indicated by flow debris that have been caught up in the branch of this mesquite tree.

So, we had now seen the evidence for what must have been an impressive flood in Puanui Channel…but, could we observe this process in action? Tropical Storm Flossie was projected to deliver up to 15 inches of rainfall on Hawaii, so we thought we might catch geology in the act…

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FLOSSIE! Just hours before impact…

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Driving out to Puanui Channel in search of flood events.

For all the hype that surrounded this tropical storm, the actual results ended up being far less impressive than our expectations. We saw about 2 inches of rainfall on Kohala — not quite enough to make Puanui flow. The main force of the storm bypassed Hawaii and delivered its precipitation to Maui and friends. Flossie did provide some much needed rain for the dry side cattle pastures, but did not move any sediment larger than clay. The BETA rocks will just have to wait to be moved from their current locations…

[Check out http://rankinstudio.com/flashfloods for more flash flood videos!]

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