How to chase waterfalls

Caption: Dr Byron Adams (UCL Earth Science) rappels down Duwee Falls on Munson Creek, Crater Lake National Park, OR, USA.

Jesse was already waist deep in rushing water and he had not even gone over the lip of the waterfall. We had all been in the water for a while and despite the blazing sun and temperatures in the 40s, I was glad that we had packed in our wetsuits. I had surveyed a lot of rivers in my time, but this one was really making us work for it. Finally, having found good footing, Jesse took the plunge and disappeared into the downpour. He made it to the bottom of the rope and swam to dry land, but as I stood there shivering in the water, I had a new concern – where am I going to set up the laser?! We had been using a laser rangefinder and global navigation satellite system to survey the flanks of Mount Mazama in the Cascade Volcanic Arc (USA) for several days and this was our last shot at getting a large, continuous dataset of the course of a river channel with the aim of understanding how rivers respond after a volcanic eruption.

Canyoneering (the sport of descending complex and hazardous canyons) is the only way to survey these hard to access portions of the landscape. It is also brilliant fun, so it is not a bad day out even if the equipment is playing up. While surveying landscapes on foot may seem antiquated in the golden era of unmanned aerial vehicles, satellite radar, and airborne lidar, it is still far more accurate, and most of the time the only way to obtain repeat data to quantify changes in the river channels. Indeed, no remote sensing technique can accurately capture the deep, narrow canyons of Mount Mazama. Therefore, conducting such surveys is often the only way to constrain river incision, which places fundamental controls on the relief of Earth’s surface, and on the pace and pattern of many geophysical hazards. 

With the influence on hazards in mind, I took these technologies to the Azores, while helping  Drs Emma Nicholson, Alex Steele, and Liz Gaunt teach GEOL0075: Geophysical Hazards Field Course. The island of São Migel in the Azores is formed by several volcanoes which have had periods of effusive and explosive eruptions, creating a complex layering of more competent (e.g., basalt) and less competent (e.g., tephra) materials. On São Migel, the students had the opportunity to measure different flow depths, channel widths and slopes along a short stretch of a river system between significant waterfalls (no canyoneering required!). While waiting out a bit of rain under one of the bridges that crosses the canyon, it became clear to us that the river was beginning to attack the foundations of the bridge, and now we had the data to explore how fast! It also became clear that even the solid looking basaltic portions of the landscape were underlain by weak tephras, making us question the stability of any portion of the landscape.

Caption: A group of Hazards MSc students survey a river channel under a bridge on São Migel in the Azores.

If you have read anything about the external forcing factors of river incision, you may have noticed that most studies focus on tectonics and climate. Tectonic rock uplift is important as it allows river systems to steepen their channels effectively (think potential energy). Variations in climate can change the discharge of river systems which influences the effectiveness of transported materials attacking the channel bed (think kinetic energy). When external factors change within a river system, the commensurate adjustments in erosion rate and channel slope start at the mouth of the river and propagate upstream like a wave. We can sometimes observe these waves in the river channel profiles as waterfalls.
While tectonics and climate often get most of the attention (I am guilty of this!), theory suggests that the materials that make of the channels of rivers have a strong control on the speed at which changes can propagate upstream, and thus set the timescale of response for variation in climate and tectonics, or rivers removing the detritus of volcanic eruptions. When materials are highly erodible, like tephra, it may be straightforward to conduct repeat surveys to constrain the important properties of the materials and better understand the process of river incision. However, often the materials we are interested in are much more resistant to erosion and repeat surveys will not be different enough to yield any new information. To overcome this challenge, I have been developing a new piece of equipment, which is designed to isolate the influence of the toughness of geological materials in the process of abrasion. Using this new bit of kit, I hope to quantify the relationship between the mechanical properties of rocks (e.g., tensile strength, elasticity, etc.) and abrasion efficiency to improve our models of river incision. Stay tuned!

Caption: The infinite flume is a bespoke machine designed to study the resistance to abrasion of geological materials. The drum of the machine (which is full of water) spins to propel glass beads into the samples contained in the two boxes attached to the perimeter.

Links:

• Dr Byron Adams's academic profile