On sinuosity (a discursive recap)

Sinuosity sneaked up on me.

A couple of miles north of Portland, Maine, there is a mudflat visible from 295, northbound, just before the highway bridge over the Presumpscot River into Falmouth. (I’ve shown it here in Slide 1, via Google Earth.) The mudflat hosts an incised channel that takes a dramatic hook near its exit into the Presumpscot itself. I drove by that mudflat almost every day for three years waiting for the pattern of that sinuous channel to change. Because that’s what sinuous channels do. They move around, they form cutoffs and oxbows, they are quintessentially dynamic landforms. Right?

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Winter storms, big tides, half a summer of heavy rain – nothing changed. (At least, nothing changed within the limit of the “65-mile-per-hour geology” that I could practice from the car. And a fine discipline that is, too. Yet another example of the wisdom bestowed upon me as a young undergraduate: geologists need designated drivers.)

So I began looking at other tidal channels, especially during runs around Portland’s Back Bay and my other usual haunts, to see if any of those showed evidence of change. None that I could see. This was getting weird. I started looking at charts and sliding around the historical-imagery bar on Google Earth.

For a long time, I regarded the immobility of tidal channels as a puzzle but not a research question. (I later learned just how late I was to this particular party: the legendary New Hampshire geologist James Walter Goldthwait published a paper in the first volume of the Proceedings of the Geological Society of America, in 1937, titled, “Unchanging meanders of tidal creeks”.) One day, in the midst of tinkering with a numerical model of a watershed I was trying to construct, and realized I could change the planform pattern of my simulated channel by adjusting two parameters: the slope of my model landscape surface, and the roughness of its terrain. (Two variables and an apparently endless variety of results? Yes, please.) Wading into rivers research means wading into one of the fastest, deepest, oldest parts of the modern geomorphology discipline. In retrospect, my approach to this project borrowed a page from the kayaker’s guide to new places – not a bad idea to sit up high, watch the rapids, eat a sandwich, and roll with someone who knows the water. To get a handle on what I was looking at, I needed a river friend. Here at Cardiff, his office is down the hall. His name is José.

The eventual paper, which printed in last week’s issue of PNAS, was a pleasure to work on – up to and including the peer-review process, which put us through our paces. Then a few lovely surprises followed. First, Danielle Venton (@DanielleVenton) caught the article in its online early-edition form and featured it in the PNAS “First Look” blog. Next, a couple of weeks later, Prof Victor Baker published an elegant companion commentary. Baker’s article is the kind of piece I’ve always wanted to write, and I hope I someday will. A mild way to describe his essay is that he puts our work in historical and philosophical perspective. Take the first two sentences: “The windings of rivers have long fascinated their human observers. For example, Aboriginal legend explains the sinuous pattern of the modern Finke River as the creation of the immense and powerful Rainbow Serpent as he emerged during the Dreamtime from deep waterholes.” Yup. Baker goes on from there to the pre-Socratic philosopher Thales and the Greek origins of the word “meander”, and includes a nod to Leonardo da Vinci, whose notebooks include sketched observations of river patterns and flows. (I love this example, in particular – a two-volume set of da Vinci’s sketchbooks is still readily accessible in the big bookcase in my parents’ house.) And to top it all off, we got a shout on the PNAS cover: “Winding patterns of rivers”.

At about the same time José and I appeared in PNAS, Exeter colleague Dr Andrew Nicholas published in Geology, delivering beautiful results from a supercomputer-powered numerical model he’s been working on that capture a spectrum of river-channel morphologies, from single-threaded to braided, to anabranching.

But extensions to sinuosity in other systems are intriguing, too. As Prof Chris Paola wrote to us, “the generic, geometric nature of the proposed model means that with a little abstraction it might well be applicable to many other fields. (Sinuous threads of moving ants? Who knows?)” As if on cue, Prof David Furbish forwarded along this article on exactly that – sinuous threads of ants – adding “the article…reminded me of your work – although any connection would be entirely heuristic.” I recommend trolling around in Google Scholar for articles on quantitative analyses of animal movement patterns. Some hidden gems in there.

Lastly, I came across the image (final slide, in the series above) of a WWI trench at Somme, collected as part of a wider WWI centenary exhibition. A haunting kind of sinuosity. I’ll leave here with the kernel of a blog entry to come, regarding the geomorphology of battlefields and the landscape legacies of modern warfare: scroll through this photo series of Germany, snapped from Allied bomber reconnaissance flights. “And now they know how many holes it takes to fill the Albert Hall…”

Thanks for reading.

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