QnAs with Gary Parker

Without water, life on Earth would cease to exist—as would many natural landscapes, according to civil engineer and geologist Gary Parker. A professor at the University of Illinois at Urbana-Champaign, Parker studies morphodynamics, or the science of how sediment movement shapes landscapes across the globe. He specializes in the transport of sediment via water bodies, resulting in features of Earth’s constantly changing surface. Morphodynamic processes are responsible for beaches on coastlines, sand dunes in deserts, and many other landforms. Parker, a member of the National Academy of Sciences since 2017, spoke with PNAS about his research as well as the relationship between nature and mathematics.

Gary Parker. Image credit: Li Zhang (photographer).

PNAS: Much of your research focuses on rivers. What inspired you to study them?

Parker: When I was a Master’s student, I went through an emotional crisis and quit. I worked at a factory for a while but knew that I couldn’t do that forever. So, I sent some applications to other schools, and a certain person who became my advisor telephoned me and said, “Gary, I’ve got a project on river meandering (1); you’re gonna love it.” He convinced me, and I’ve been researching river meandering ever since.

PNAS: What is river meandering?

Parker: There’s a natural tendency for rivers to develop bars, which are local deposits that scale with the width of a river and alternate back and forth. These bars tend to grow into a meandering shape. That’s the natural state of most rivers.

PNAS: Speaking of shape, how do your analyses explain the shaping of Earth’s surface?

Parker: Take continental shelves, for example. They’re somehow associated with the interaction of water and sediment, and are kind of like shoulders that surround almost all continents. We’d like to know why these shoulders are there. Then we have a meandering river; we’d like to know why it’s there and how it forms. Another question is how sediment fills in a lake over time. One way to research these topics is through fieldwork: We go out, look at a phenomenon, get data, and find out what we can directly from the phenomenon itself. Another approach is experiments, where we have a simplified version in the laboratory and study it. And then there’s mathematical theory, where we try to put together the equations that govern a phenomenon, like meandering or continental shelf formation, and see if we can explain what we see in nature with mathematics. Although I’m interested in all of these approaches, I tend to be on the theoretical side.

PNAS: Why are you drawn to the mathematical aspect?

Parker: To most people, equations sort of look like scribbles on a piece of paper. For me, if a phenomenon described in equations jumps out and says, “Hey, this is how the system works,” then the equations acquire a beauty that’s similar to the phenomenon itself. The types of things that I work with, like river networks and coastlines, are intrinsically beautiful, so it’s compelling to find the mathematics that parallel the beauty of nature.

PNAS: We know that climate change affects nature in various ways. How does it influence river morphodynamics?

Parker: We’re learning more and more that rivers are the way they are because they move, and that movement is strongly affected by factors like change in precipitation. In particular, there’s a river in Minnesota we like to study, appropriately called the Minnesota River. The mean rainfall there has increased somewhat over the last century, and the rainfalls have also become more intense (https://arcgis.dnr.state.mn.us/ewr/climatetrends/). This makes a difference for the adjacent farmland; it makes a difference for floods (2). Climate change also comes into play indirectly through sea level rise, which greatly affects the Mississippi Delta—right now, it’s sediment-starved. We’ve discussed plans to open up the Mississippi River and feed more sediment into the delta, but sea level rise is drowning that sediment (3).

PNAS: Why is sediment important? Doesn’t it pollute water?

Parker: Sediment is a crucial part of ecosystems. The most common sediment in rivers is made of silicate minerals. And the form that it’s in at the bottom of rivers is not only perfectly safe, but necessary for ecosystems as we know them. If we build a dam, and downstream of it, all of the sediment gets stripped off the bedrock, then we no longer have habitats for fish or other organisms.

PNAS: Your Inaugural Article (4) incorporates modeling of sediment transport and morphodynamics to explain how continental shelves are formed. What was the impetus for this study?

Parker: The standard explanation for the shelf on the East Coast of the United States is that every 100,000 years or so, the sea level went up and down by about 100 meters, which helped build it out. But we’re beginning to see evidence that continental shelves are being built today without requiring a change in sea level. Around 1999, I suggested the idea that continental shelves could actively build out if rivers were supplying enough mud. Finally, I teamed up with Toshiki Iwasaki, a postdoctoral researcher, and created a model showing that if we deliver mud to a freshwater shoreline, the mud-laden freshwater becomes heavier than lake freshwater and dives down to the bottom of the lake. And this is exactly what we see in a number of locations around the world, such as the north shoulder of the Gulf of Mexico and the northwestern side of the Amazon River. The key thing is salt; dissolved salt makes seawater heavy. Even fresh river water laden with sediment is usually lighter than seawater—it can float on top of seawater rather than dive down and run out into deep water.

PNAS: What was the most interesting finding from your Inaugural Article (4)?

Parker: If you started off with a coast that had no shelf, it would just drop straight off into deep water. Sediment builds up from deep water into shallow water until it hits something we call wave base, where waves can affect it. The waves can then sweep the sediment out to sea, which creates an area of shallower water with a bench. This bench is a patch of sloped terrain beneath the water, and it’s also a type of continental shelf. Over time, the bench migrates into the sea, as an edge of it goes deep into the water. Finding that migrating bench was very exciting.

PNAS: Do you have other exciting projects in the works?

Parker: About a year ago, I was with a postdoc, and we were stumbling around trying to find a site in the Sierra Nevada but couldn’t get to it because the trail was closed. On the way back, we drove through Death Valley National Park, and there’s an overlook called Father Crowley, which shows a canyon called Rainbow Canyon, which flows into the Panamint Valley. It was just so gorgeous that we developed a theory of how it formed and wrote a paper about it (5).